This RFC is to illustrate how attach disk to vm works and what the API looks like.

Use case:
    1. Guest wants to attach a volume which contains data.
    2. Guest is running out of storage space, attach an empty volume to extend storage space.

Flow:
    1. Create a *float* volume in a storage pool.(float means this volume will not be deleted when vm's deleted, it is independent from vm)
        Only *float* volume can be attached to vm.(to solve volume ref count problem), All volumes created from storage tab is float ones,
        because embedded storage is created when vm's created.
 
        POST /storagepools/pool-1/storagevolumes/
           {'float': True, 'size':10G, 'name': 'a-float-volume'}
        GET /storagepools/pool-1/storagevolumes/a-float-volume
           {'float': True, 'ref_count':0, ...}

    2. On vm tab, list all availabe float volumes, currently only support ref_count=0 volumes, select one and attach it to vm.
        GET /storagepools/pool-1/storagevolumes?float=True&ref_count=0
        GET /storagepools/pool-2/storagevolumes?float=True&ref_count=0
        ...
        POST /vms/vm-1/storages/
        {'storagepool': 'pool-1', 'storagevolume':'a-float-volume', 'dev': 'hda', 'type':'disk'}

    3. Detaching a disk from a vm, NOTE: this will make no change to content in the volume, just make it isolated from vm, if you want to delete storage, go to storage tab.
        DELETE /vms/vm-1/storages/hda

NOTE:
    This means we support disks from different storagepool,
    Temporarily we don't support share disk (in the future it will be handled in a single feature).
    VM deletion will not involve float disk.
    As PCI hotplug already supported, we can add disk to a running vm.

Thanks to Mark Wu to contribute idea of float to solve volume life cycle problem. Part of the inner implementation already be done by Rodrigo and Daniel, I will try to extend their implementation for hard disk.