bdrv_root_attach_child() requires callers to hold the AioContext lock
for child_bs. Take it in block_job_add_bdrv() before calling the
function.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20230605085711.21261-10-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>