Hello,
installed a CentOS 6.5 x86_64 vm from iso (create VM and then run once connecting CD iso).

System reboot several times and power on from web admin gui and all seems ok.

At a certain point I upload guest-tools iso to my engine for future windows VMs.
Taken ovirt-guest-tools-3.5_5.iso and try to mount it

In engine the downloaded file seems ok and I can loop mount it for test.
[root@ovirtmgr ~]# mount -o loop ovirt-guest-tools-3.5_5.iso /media
[root@ovirtmgr ~]# ll /media
total 19382
dr-xr-xr-x. 4 root root     2048 Jul  7 13:21 bin
dr-xr-xr-x. 4 root root     2048 Jul  6 17:25 drivers
-r--r--r--. 1 root root 19839504 Jul 17 12:49 ovirt-guest-tools-setup.exe
-r--r--r--. 1 root root       88 Jul  7 13:18 version.txt
-r--r--r--. 1 root root     2266 Jul  2 16:40 win-driver-license.txt
[root@ovirtmgr ~]# umount /media

[root@ovirtmgr ~]# md5sum ovirt-guest-tools-3.5_5.iso
ad98fc5db09183acd1620b5effb5e151  ovirt-guest-tools-3.5_5.iso

Also the uploaded file seems ok
[root@ovirtmgr ovirt-engine]# md5sum /var/lib/exports/iso/3b23161a-18a3-4cfa-bc78-f4fe2052b19e/images/11111111-1111-1111-1111-111111111111/ovirt-guest-tools-3.5_5.iso
ad98fc5db09183acd1620b5effb5e151  /var/lib/exports/iso/3b23161a-18a3-4cfa-bc78-f4fe2052b19e/images/11111111-1111-1111-1111-111111111111/ovirt-guest-tools-3.5_5.iso   

If I try to connect cd to the running VM from web admin portal I get a window with the error:

Error while executing action Change CD: Internal Engine Error

In events:
Failed to change disk in VM c65new (Host: ovnode04, User: admin).

In engine.log
2014-09-23 00:33:43,022 INFO  [org.ovirt.engine.core.bll.ChangeDiskCommand] (ajp--127.0.0.1-8702-5) [22041
702] Running command: ChangeDiskCommand internal: false. Entities affected :  ID: 0ce8ebc0-8464-4e9a-b382-1836234b3560 Type: VMAction group CHANGE_VM_CD with role type USER
2014-09-23 00:33:43,028 INFO  [org.ovirt.engine.core.vdsbroker.vdsbroker.IsoPrefixVDSCommand] (ajp--127.0.0.1-8702-5) [22041702] START, IsoPrefixVDSCommand(HostName = ovnode04, HostId = 36fec87b-c21f-4157-ab2f-434b67c05cb9, storagePoolId=4512e567-f94e-476a-a050-6cd0a15e260a), log id: 26c82fe2
2014-09-23 00:33:43,028 INFO  [org.ovirt.engine.core.vdsbroker.vdsbroker.IsoPrefixVDSCommand] (ajp--127.0.0.1-8702-5) [22041702] FINISH, IsoPrefixVDSCommand, return: /rhev/data-center/mnt/ovirtmgr.localdomain.local:_var_lib_exports_iso/3b23161a-18a3-4cfa-bc78-f4fe2052b19e/images/11111111-1111-1111-1111-111111111111, log id: 26c82fe2
2014-09-23 00:33:43,029 INFO  [org.ovirt.engine.core.vdsbroker.vdsbroker.ChangeDiskVDSCommand] (ajp--127.0.0.1-8702-5) [22041702] START, ChangeDiskVDSCommand(HostName = ovnode04, HostId = 36fec87b-c21f-4157-ab2f-434b67c05cb9, vmId=0ce8ebc0-8464-4e9a-b382-1836234b3560, diskPath=/rhev/data-center/mnt/ovirtmgr.localdomain.local:_var_lib_exports_iso/3b23161a-18a3-4cfa-bc78-f4fe2052b19e/images/11111111-1111-1111-1111-111111111111/ovirt-guest-tools-3.5_5.iso), log id: 12ce4870
2014-09-23 00:33:43,060 ERROR [org.ovirt.engine.core.vdsbroker.vdsbroker.ChangeDiskVDSCommand] (ajp--127.0.0.1-8702-5) [22041702] Failed in ChangeDiskVDS method, for vds: ovnode04; host: 192.168.1.74
2014-09-23 00:33:43,061 ERROR [org.ovirt.engine.core.vdsbroker.vdsbroker.ChangeDiskVDSCommand] (ajp--127.0.0.1-8702-5) [22041702] Command ChangeDiskVDSCommand(HostName = ovnode04, HostId = 36fec87b-c21f-4157-ab2f-434b67c05cb9, vmId=0ce8ebc0-8464-4e9a-b382-1836234b3560, diskPath=/rhev/data-center/mnt/ovirtmgr.localdomain.local:_var_lib_exports_iso/3b23161a-18a3-4cfa-bc78-f4fe2052b19e/images/11111111-1111-1111-1111-111111111111/ovirt-guest-tools-3.5_5.iso) execution failed. Exception: NullPointerException:
2014-09-23 00:33:43,063 INFO  [org.ovirt.engine.core.vdsbroker.vdsbroker.ChangeDiskVDSCommand] (ajp--127.0.0.1-8702-5) [22041702] FINISH, ChangeDiskVDSCommand, log id: 12ce4870
2014-09-23 00:33:43,063 ERROR [org.ovirt.engine.core.bll.ChangeDiskCommand] (ajp--127.0.0.1-8702-5) [22041702] Command org.ovirt.engine.core.bll.ChangeDiskCommand throw Vdc Bll exception. With error message VdcBLLException: java.lang.NullPointerException (Failed with error ENGINE and code 5001)
2014-09-23 00:33:43,069 ERROR [org.ovirt.engine.core.bll.ChangeDiskCommand] (ajp--127.0.0.1-8702-5) [22041702] Transaction rolled-back for command: org.ovirt.engine.core.bll.ChangeDiskCommand.
2014-09-23 00:33:43,083 ERROR [org.ovirt.engine.core.dal.dbbroker.auditloghandling.AuditLogDirector] (ajp--127.0.0.1-8702-5) [22041702] Correlation ID: 22041702, Job ID: a6be11b2-bb67-49ec-b3fc-695e8eab36b2, Call Stack: null, Custom Event ID: -1, Message: Failed to change disk in VM c65new (Host: ovnode04, User: admin).

No errors in vdsm.log of host and in VM logfile under /var/log/libvirt/qemu on hypervisor.

The Vm runs with this command line generated by oVirt

qemu     31193     1  0 Sep16 ?        01:36:41 /usr/libexec/qemu-kvm -name c65new -S -M rhel6.5.0 -cpu Nehalem -enable-kvm -m 2048 -realtime mlock=off -smp 2,maxcpus=16,sockets=16,cores=1,threads=1 -uuid 0ce8ebc0-8464-4e9a-b382-1836234b3560 -smbios type=1,manufacturer=oVirt,product=oVirt Node,version=6-5.el6.centos.11.2,serial=9B1B2F84-3CB9-11DE-B5A1-001517B493E8,uuid=0ce8ebc0-8464-4e9a-b382-1836234b3560 -nodefconfig -nodefaults -chardev socket,id=charmonitor,path=/var/lib/libvirt/qemu/c65new.monitor,server,nowait -mon chardev=charmonitor,id=monitor,mode=control -rtc base=2014-09-16T00:22:11,driftfix=slew -no-kvm-pit-reinjection -no-shutdown -device piix3-usb-uhci,id=usb,bus=pci.0,addr=0x1.0x2 -device virtio-scsi-pci,id=scsi0,bus=pci.0,addr=0x4 -device virtio-serial-pci,id=virtio-serial0,max_ports=16,bus=pci.0,addr=0x5 -drive if=none,media=cdrom,id=drive-ide0-1-0,readonly=on,format=raw,serial= -device ide-drive,bus=ide.1,unit=0,drive=drive-ide0-1-0,id=ide0-1-0 -drive file=/rhev/data-center/4512e567-f94e-476a-a050-6cd0a15e260a/346732c8-e1e9-49de-9ca4-63f4477ef6dd/images/f9c73080-8653-44bd-9bd9-acc7a9925971/72821a56-1806-4b8f-bd73-7b5ef4f3c12f,if=none,id=drive-scsi0-0-0-0,format=qcow2,serial=f9c73080-8653-44bd-9bd9-acc7a9925971,cache=none,werror=stop,rerror=stop,aio=native -device scsi-hd,bus=scsi0.0,channel=0,scsi-id=0,lun=0,drive=drive-scsi0-0-0-0,id=scsi0-0-0-0,bootindex=1 -netdev tap,fd=26,id=hostnet0,vhost=on,vhostfd=27 -device virtio-net-pci,netdev=hostnet0,id=net0,mac=00:01:a4:a3:42:01,bus=pci.0,addr=0x3 -chardev socket,id=charchannel0,path=/var/lib/libvirt/qemu/channels/0ce8ebc0-8464-4e9a-b382-1836234b3560.com.redhat.rhevm.vdsm,server,nowait -device virtserialport,bus=virtio-serial0.0,nr=1,chardev=charchannel0,id=channel0,name=com.redhat.rhevm.vdsm -chardev socket,id=charchannel1,path=/var/lib/libvirt/qemu/channels/0ce8ebc0-8464-4e9a-b382-1836234b3560.org.qemu.guest_agent.0,server,nowait -device virtserialport,bus=virtio-serial0.0,nr=2,chardev=charchannel1,id=channel1,name=org.qemu.guest_agent.0 -chardev spicevmc,id=charchannel2,name=vdagent -device virtserialport,bus=virtio-serial0.0,nr=3,chardev=charchannel2,id=channel2,name=com.redhat.spice.0 -chardev pty,id=charconsole0 -device virtconsole,chardev=charconsole0,id=console0 -spice port=5900,tls-port=5901,addr=192.168.1.74,x509-dir=/etc/pki/vdsm/libvirt-spice,tls-channel=main,tls-channel=display,tls-channel=inputs,tls-channel=cursor,tls-channel=playback,tls-channel=record,tls-channel=smartcard,tls-channel=usbredir,seamless-migration=on -k en-us -vga qxl -global qxl-vga.ram_size=67108864 -global qxl-vga.vram_size=33554432 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x6

It seems to be a general problem. Tried also to mount the iso of CentOS 6.5 that I used to install the VM itself but I get the same error.

Donna if it is somehow related also with what I previously wrote about cd and install in 3.5 rc2; see
http://lists.ovirt.org/pipermail/users/2014-September/027465.html

Gianluca