bdrv_attach_child() requires that the caller holds the AioContext lock
for the new child node. Take it in bdrv_open_child() and document that
the caller must not hold any AioContext apart from the main AioContext.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20230605085711.21261-5-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>