Keep the NBD server always in the same AIO context as the exported BDS
by calling bdrv_add_aio_context_notifier() and implementing the required
callbacks.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>