qcow2: Fix open with 'file' in iothread

qcow2_open() doesn't work correctly when opening the 'file' child moves
bs to an iothread, for several reasons:

- It uses BDRV_POLL_WHILE() to wait for the qcow2_open_entry()
  coroutine, which involves dropping the AioContext lock for bs when it
  is not in the main context - but we don't hold it, so this crashes.

- It runs the qcow2_open_entry() coroutine in the current thread instead
  of the new AioContext of bs.

- qcow2_open_entry() doesn't notify the main loop when it's done.

This patches fixes these issues around delegating work to a coroutine.
Temporarily dropping the main AioContext lock is not necessary because
we know we run in the main thread.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230525124713.401149-7-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -3646,6 +3646,9 @@ done:
  * BlockdevRef.
  *
  * The BlockdevRef will be removed from the options QDict.
+ *
+ * @parent can move to a different AioContext in this function. Callers must
+ * make sure that their AioContext locking is still correct after this.
  */
 BdrvChild *bdrv_open_child(const char *filename,
                            QDict *options, const char *bdref_key,
@@ -3670,6 +3673,9 @@ BdrvChild *bdrv_open_child(const char *filename,
 
 /*
  * Wrapper on bdrv_open_child() for most popular case: open primary child of bs.
+ *
+ * @parent can move to a different AioContext in this function. Callers must
+ * make sure that their AioContext locking is still correct after this.
  */
 int bdrv_open_file_child(const char *filename,
                          QDict *options, const char *bdref_key,