nbd: drop unused nbd_start_negotiate() aio_context argument

aio_context is always NULL, so drop it.

Suggested-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20230830224802.493686-3-stefanha@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>