block: Make format drivers use child_of_bds

Commonly, they need to pass the BDRV_CHILD_IMAGE set as the
BdrvChildRole; but there are exceptions for drivers with external data
files (qcow2 and vmdk).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200513110544.176672-26-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qed.c

@@ -547,8 +547,8 @@ static int bdrv_qed_open(BlockDriverState *bs, QDict *options, int flags,
         .ret = -EINPROGRESS
     };
 
-    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_file, 0,
-                               false, errp);
+    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_of_bds,
+                               BDRV_CHILD_IMAGE, false, errp);
     if (!bs->file) {
         return -EINVAL;
     }