blkdebug: Don't confuse image as backing file

The word "backing file" nowadays refers to the backing_hd in the
external snapshot sense (i.e. bs->backing_hd), instead of the file sense
(bs->file). Correct the comment to use the right term.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/blkdebug.c

@@ -426,7 +426,7 @@ static int blkdebug_open(BlockDriverState *bs, QDict *options, int flags,
     /* Set initial state */
     s->state = 1;
 
-    /* Open the backing file */
+    /* Open the image file */
     bs->file = bdrv_open_child(qemu_opt_get(opts, "x-image"), options, "image",
                                bs, &child_file, false, &local_err);
     if (local_err) {