Rename rbd.py to rbd_utils.py in libvirt driver directory

In libvirt driver directory, rbd.py confict with global rbd library
which is imported in rbd.py, so we rename rbd.py to rbd_utils.py.

Change-Id: Ib62e430e678fe09c4a8475a636a8ecc11a194f5c
Closes-Bug: #1352595
libvirt: support live migration with shared instances dir

If instance path is shared between compute nodes (e.g. over NFS) and image
backend uses files placed in the instance directory (e.g. Raw, Qcow2), live
migration should consider block storage of the instances shared.

Change-Id: I913c57152974562765502ab57be63ccb6dfe5208
Closes-Bug: #1351002