discard rbd error output when not relevant in qemu-iotests

Suppress rbd progress messages with --no-progress so they are not
confused with an error output when comparing test results ( progress is
displayed on stderr ).

Signed-off-by: Loic Dachary <loic@dachary.org>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

tests/qemu-iotests/common.rc

@@ -203,7 +203,7 @@ _cleanup_test_img()
             ;;
 
         rbd)
-            rbd rm "$TEST_DIR/t.$IMGFMT" > /dev/null
+            rbd --no-progress rm "$TEST_DIR/t.$IMGFMT" > /dev/null
             ;;
 
         sheepdog)