core: Use correct storage validation for MoveVM

Fix MoveVMCommand to use the new (correct)
StorageDomainValidator.hasSpaceForClonedDisks(Iterable<DiskImage>) to
validate the destination domain has enough space.

Change-Id: I7f455ff16d11622be4e28ea84180551237a80db1
Signed-off-by: Allon Mureinik <amureini@redhat.com>