block: Introduce BDS.file_child

Store the BdrvChild for bs->file. At this point, bs->file_child->bs just
duplicates the bs->file pointer. Later, it will completely replace it.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NAlberto Garcia <berto@igalia.com>
Reviewed-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NJeff Cody <jcody@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>

block.c

@@ -1487,11 +1487,17 @@ static int bdrv_open_inherit(BlockDriverState **pbs, const char *filename,
 
         assert(file == NULL);
         bs->open_flags = flags;
-        ret = bdrv_open_image(&file, filename, options, "file",
-                              bs, &child_file, true, &local_err);
-        if (ret < 0) {
+
+        bs->file_child = bdrv_open_child(filename, options, "file", bs,
+                                         &child_file, true, &local_err);
+        if (local_err) {
+            ret = -EINVAL;
             goto fail;
         }
+
+        if (bs->file_child) {
+            file = bs->file_child->bs;
+        }
     }
 
     /* Image format probing */

include/block/block_int.h

@@ -381,6 +381,7 @@ struct BlockDriverState {
     BlockDriverState *backing_hd;
     BdrvChild *backing_child;
     BlockDriverState *file;
+    BdrvChild *file_child;
 
     NotifierList close_notifiers;