Allow Cinder to specify file format for NFS/GlusterFS

These drivers now have the option of passing a QCOW2 file rather
than a raw file as the volume to attach to the VM.

Required by blueprint qemu-assisted-snapshots

Change-Id: I23929538c7dad65035db5c38ff6e52348925fa27
Merge "Allow Cinder to specify file format for NFS/GlusterFS"