storage: Handle failure from refreshVol

storage: Handle failure from refreshVol

Commit id '155ca616' added the 'refreshVol' API. In an NFS root-squash
environment it was possible that if the just created volume from XML wasn't
properly created with the right uid/gid and/or mode, then the followup
refreshVol will fail to open the volume in order to get the allocation/
capacity values. This would leave the volume still on the server and
cause a libvirtd crash because 'voldef' would be in the pool list, but
the cleanup code would free it.