Is /posix mounted?

    root@host008:/tmp mount |grep fuse
    root@host008:/tmp

Nope.
Add Storage Domain via Web Panel with parameters:

    Name: MooseFS
    Domain Function / Storage Type: Data / POSIX Compliant FS
    Use Host: HOST008
    Path: mfsmount
    VFS Type: fuse
    Mount Options: mfsmaster=mfsmaster,mfsport=9421,mfssubfolder=/posix,_netdev

Debug logs from task:

    JsonRpc (StompReactor)::DEBUG::2015-04-02 08:52:58,231::stompReactor::98::Broker.StompAdapter::(handle_frame) Handling message <StompFrame command='SEND'>
    JsonRpcServer::DEBUG::2015-04-02 08:52:58,232::__init__::506::jsonrpc.JsonRpcServer::(serve_requests) Waiting for request
    Thread-549209::DEBUG::2015-04-02 08:52:58,232::__init__::469::jsonrpc.JsonRpcServer::(_serveRequest) Calling 'StoragePool.connectStorageServer' in bridge with {'connectionParams': [{'password': '', 'id': '00000000-0000-0000-0000-000000000000', 'connection': 'mfsmount', 'mnt_options': 'mfsmaster=mfsmaster,mfsport=9421,mfssubfolder=/posix,_netdev', 'user': '', 'tpgt': '1', 'vfs_type': 'fuse', 'iqn': '', 'port': ''}], 'storagepoolID': '00000000-0000-0000-0000-000000000000', 'domainType': 6}
    Thread-549209::DEBUG::2015-04-02 08:52:58,234::task::595::Storage.TaskManager.Task::(_updateState) Task=`9bb09583-d8f7-4189-b9ab-81b925f8fc13`::moving from state init -> state preparing
    Thread-549209::INFO::2015-04-02 08:52:58,234::logUtils::44::dispatcher::(wrapper) Run and protect: connectStorageServer(domType=6, spUUID='00000000-0000-0000-0000-000000000000', conList=[{'iqn': '', 'port': '', 'connection': 'mfsmount', 'mnt_options': 'mfsmaster=mfsmaster,mfsport=9421,mfssubfolder=/posix,_netdev', 'user': '', 'tpgt': '1', 'vfs_type': 'fuse', 'password': '******', 'id': '00000000-0000-0000-0000-000000000000'}], options=None)
    Thread-549209::DEBUG::2015-04-02 08:52:58,237::fileUtils::142::Storage.fileUtils::(createdir) Creating directory: /rhev/data-center/mnt/mfsmount
    Thread-549209::WARNING::2015-04-02 08:52:58,237::fileUtils::149::Storage.fileUtils::(createdir) Dir /rhev/data-center/mnt/mfsmount already exists
    Thread-549209::DEBUG::2015-04-02 08:52:58,238::mount::227::Storage.Misc.excCmd::(_runcmd) /usr/bin/sudo -n /bin/mount -t fuse -o mfsmaster=mfsmaster,mfsport=9421,mfssubfolder=/posix,_netdev mfsmount /rhev/data-center/mnt/mfsmount (cwd None)
    JsonRpc (StompReactor)::DEBUG::2015-04-02 08:52:58,271::stompReactor::98::Broker.StompAdapter::(handle_frame) Handling message <StompFrame command='SEND'>
    JsonRpcServer::DEBUG::2015-04-02 08:52:58,273::__init__::506::jsonrpc.JsonRpcServer::(serve_requests) Waiting for request
    Thread-549210::DEBUG::2015-04-02 08:52:58,276::stompReactor::163::yajsonrpc.StompServer::(send) Sending response
    JsonRpc (StompReactor)::DEBUG::2015-04-02 08:52:58,279::stompReactor::98::Broker.StompAdapter::(handle_frame) Handling message <StompFrame command='SEND'>
    JsonRpcServer::DEBUG::2015-04-02 08:52:58,280::__init__::506::jsonrpc.JsonRpcServer::(serve_requests) Waiting for request
    Thread-549211::DEBUG::2015-04-02 08:52:58,282::stompReactor::163::yajsonrpc.StompServer::(send) Sending response
    Thread-549209::ERROR::2015-04-02 08:52:58,523::hsm::2424::Storage.HSM::(connectStorageServer) Could not connect to storageServer
    Traceback (most recent call last):
    File "/usr/share/vdsm/storage/hsm.py", line 2421, in connectStorageServer
        conObj.connect()
    File "/usr/share/vdsm/storage/storageServer.py", line 222, in connect
        self.getMountObj().getRecord().fs_file)
    File "/usr/share/vdsm/storage/mount.py", line 278, in getRecord
        (self.fs_spec, self.fs_file))
    OSError: [Errno 2] Mount of `mfsmount` at `/rhev/data-center/mnt/mfsmount` does not exist
    Thread-549209::DEBUG::2015-04-02 08:52:58,524::hsm::2443::Storage.HSM::(connectStorageServer) knownSDs: {2df46204-217e-416f-a072-ab8ef17cd8d2: storage.nfsSD.findDomain, 316c3e1c-4e61-4b0a-b2f6-63cc22d3ab25: storage.nfsSD.findDomain, 6c348f77-bb02-4135-b629-2c9cacb0b85c: storage.nfsSD.findDomain, 00697722-a1ce-4911-a84c-c4688e5076fe: storage.nfsSD.findDomain, 6ac038d7-969d-45b5-be5f-c58a66a78a90: storage.nfsSD.findDomain}
    Thread-549209::INFO::2015-04-02 08:52:58,524::logUtils::47::dispatcher::(wrapper) Run and protect: connectStorageServer, Return response: {'statuslist': [{'status': 100, 'id': '00000000-0000-0000-0000-000000000000'}]}
    Thread-549209::DEBUG::2015-04-02 08:52:58,525::task::1191::Storage.TaskManager.Task::(prepare) Task=`9bb09583-d8f7-4189-b9ab-81b925f8fc13`::finished: {'statuslist': [{'status': 100, 'id': '00000000-0000-0000-0000-000000000000'}]}
    Thread-549209::DEBUG::2015-04-02 08:52:58,525::task::595::Storage.TaskManager.Task::(_updateState) Task=`9bb09583-d8f7-4189-b9ab-81b925f8fc13`::moving from state preparing -> state finished
    Thread-549209::DEBUG::2015-04-02 08:52:58,526::resourceManager::940::Storage.ResourceManager.Owner::(releaseAll) Owner.releaseAll requests {} resources {}
    Thread-549209::DEBUG::2015-04-02 08:52:58,526::resourceManager::977::Storage.ResourceManager.Owner::(cancelAll) Owner.cancelAll requests {}
    Thread-549209::DEBUG::2015-04-02 08:52:58,526::task::993::Storage.TaskManager.Task::(_decref) Task=`9bb09583-d8f7-4189-b9ab-81b925f8fc13`::ref 0 aborting False
    Thread-549209::DEBUG::2015-04-02 08:52:58,527::__init__::500::jsonrpc.JsonRpcServer::(_serveRequest) Return 'StoragePool.connectStorageServer' in bridge with [{'status': 100, 'id': '00000000-0000-0000-0000-000000000000'}]
    Thread-549209::DEBUG::2015-04-02 08:52:58,527::stompReactor::163::yajsonrpc.StompServer::(send) Sending response
    Thread-29::DEBUG::2015-04-02 08:52:59,236::domainMonitor::209::Storage.DomainMonitorThread::(_monitorDomain) Refreshing domain 316c3e1c-4e61-4b0a-b2f6-63cc22d3ab25

Really? Incorrect mount options? Trying mount from hand... But first, check for mounts:

    root@host008:/tmp mount |grep fuse
    mfsmaster:9421 on /rhev/data-center/mnt/mfsmount type fuse.mfs (rw,allow_other)

Oh! It is already mounted! So oVirt mounted it correctly, but cannot use mountpoint? Hm, permissions?

    root@host008:/tmp ls -al /rhev/data-center/mnt/mfsmount/
    total 4
    drwxr-xr-x 2 vdsm kvm    0 Apr  1 14:57 .
    drwxr-xr-x 8 vdsm kvm 4096 Apr  2 08:43 ..

Looks fine... Ok, try to unmount and mount manually.

    root@host008:/tmp umount /rhev/data-center/mnt/mfsmount/
    root@host008:/tmp mount |grep fuse
    root@host008:/tmp mount -t fuse -o mfsmaster=mfsmaster,mfsport=9421,mfssubfolder=/posix,_netdev mfsmount /rhev/data-center/mnt/mfsmount
    mfsmaster accepted connection with parameters: read-write,restricted_ip ; root mapped to root:root
    root@host008:/tmp mount |grep fuse
    mfsmaster:9421 on /rhev/data-center/mnt/mfsmount type fuse.mfs (rw,allow_other)
    root@host008:/tmp ls -al /rhev/data-center/mnt/mfsmount/
    total 4
    drwxr-xr-x 2 vdsm kvm    0 Apr  1 14:57 .
    drwxr-xr-x 8 vdsm kvm 4096 Apr  2 08:43 ..

Ok, but I'm root. Try to do it as vdsm user.

    root@host008:/tmp cat /etc/passwd /etc/group |grep :36:
    vdsm:x:36:36:Node Virtualization Manager:/var/lib/vdsm:/bin/bash
    kvm:x:36:qemu,sanlock
    root@host008:/tmp umount /rhev/data-center/mnt/mfsmount/
    root@host008:/tmp su vdsm
    vdsm@host008:/tmp /usr/bin/sudo -n /bin/mount -t fuse -o mfsmaster=mfsmaster,mfsport=9421,mfssubfolder=/posix,_netdev mfsmount /rhev/data-center/mnt/mfsmount
    mfsmaster accepted connection with parameters: read-write,restricted_ip ; root mapped to root:root
    vdsm@host008:/tmp ls -al /rhev/data-center/mnt/mfsmount/
    total 4
    drwxr-xr-x 2 vdsm kvm    0 Apr  1 14:57 .
    drwxr-xr-x 8 vdsm kvm 4096 Apr  2 08:43 ..
    vdsm@host008:/tmp mkdir /rhev/data-center/mnt/mfsmount/ovirt-storage-test
    vdsm@host008:/tmp ls -al /rhev/data-center/mnt/mfsmount/
    total 4
    drwxr-xr-x 3 vdsm kvm    0 Apr  2  2015 .
    drwxr-xr-x 8 vdsm kvm 4096 Apr  2 08:43 ..
    drwxr-xr-x 2 vdsm kvm    0 Apr  2  2015 ovirt-storage-test