block: Fix AioContext locking in bdrv_open_backing_file()

bdrv_set_backing() requires the caller to hold the AioContext lock for
@backing_hd. Take it in bdrv_open_backing_file() before calling the
function.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20230605085711.21261-9-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>