blockdev: move BDRV_O_NO_BACKING option forward

For both external_snapshot_prepare and qmp_drive_mirror, we eventually
append the option BDRV_O_NO_BACKING. However, we generally do so after
we create the image.

To accommodate image creation wanting to verify that a backing file
exists or not, add this option prior to create to override checking
the existence of the backing file. This prevents QEMU from trying to
re-open a backing file that's already in use (thanks to qcow2 locking).
Signed-off-by: NJohn Snow <jsnow@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

blockdev.c

@@ -1710,7 +1710,8 @@ static void external_snapshot_prepare(BlkActionState *common,
         }
 
         flags = state->old_bs->open_flags;
-        flags &= ~(BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING | BDRV_O_COPY_ON_READ);
+        flags &= ~(BDRV_O_SNAPSHOT | BDRV_O_COPY_ON_READ);
+        flags |= BDRV_O_NO_BACKING;
 
         /* create new image w/backing file */
         mode = s->has_mode ? s->mode : NEW_IMAGE_MODE_ABSOLUTE_PATHS;
@@ -1735,8 +1736,6 @@ static void external_snapshot_prepare(BlkActionState *common,
             qdict_put_str(options, "node-name", snapshot_node_name);
         }
         qdict_put_str(options, "driver", format);
-
-        flags |= BDRV_O_NO_BACKING;
     }
 
     state->new_bs = bdrv_open(new_image_file, snapshot_ref, options, flags,
@@ -3548,6 +3547,9 @@ void qmp_drive_mirror(DriveMirror *arg, Error **errp)
         backing_mode = MIRROR_OPEN_BACKING_CHAIN;
     }
 
+    /* Don't open backing image in create() */
+    flags |= BDRV_O_NO_BACKING;
+
     if ((arg->sync == MIRROR_SYNC_MODE_FULL || !source)
         && arg->mode != NEW_IMAGE_MODE_EXISTING)
     {
@@ -3587,8 +3589,7 @@ void qmp_drive_mirror(DriveMirror *arg, Error **errp)
     /* Mirroring takes care of copy-on-write using the source's backing
      * file.
      */
-    target_bs = bdrv_open(arg->target, NULL, options,
-                          flags | BDRV_O_NO_BACKING, errp);
+    target_bs = bdrv_open(arg->target, NULL, options, flags, errp);
     if (!target_bs) {
         goto out;
     }