rbd: check for valid UUID instead of name

It seems to be possible that the UUID was found, but the name is not
set. Checking on UUID makes the CreateVolumeGroup operation more
idempotent.

Signed-off-by: Niels de Vos <ndevos@ibm.com>

internal/rbd/manager.go

@@ -192,7 +192,7 @@ func (mgr *rbdManager) CreateVolumeGroup(ctx context.Context, name string) (type
 	}
 
 	var uuid string
-	if vgData != nil && vgData.GroupName != "" {
+	if vgData != nil && vgData.GroupUUID != "" {
 		uuid = vgData.GroupUUID
 	} else {
 		log.DebugLog(ctx, "the journal does not contain a reservation for a volume group with name %q yet", name)