don't use vdsm backend when there is not valid vol or imgUUID

Change-Id: Idcd181d260f4f20676a4124b899575b42935753c
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1171452
Signed-off-by: Jiri Moskovcak <jmoskovc@redhat.com>