nbd/server: Add va_list form of nbd_negotiate_send_rep_err()

This will be useful for the next patch.

Based on a patch by Vladimir Sementsov-Ogievskiy

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20180110230825.18321-5-eblake@redhat.com>