Fix BlockdevSnapshot in case of non lvm-based disk

In case of non lvm-based disk (i.e. drbd, plain disk template)
snapshot is currently not supported. The RPC will fail reporting the
disk's logical_id (and not unique_id since only BlockDev has one)
and its dev_type.
Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>

lib/backend.py

@@ -3072,7 +3072,7 @@ def BlockdevSnapshot(disk):
       _Fail("Cannot find block device %s", disk)
   else:
     _Fail("Cannot snapshot non-lvm block device '%s' of type '%s'",
-          disk.unique_id, disk.dev_type)
+          disk.logical_id, disk.dev_type)
 
 
 def BlockdevSetInfo(disk, info):