nbd/client: Pull out oldstyle size determination

Another refactoring creating nbd_negotiate_finish_oldstyle()
for further reuse during 'qemu-nbd --list'.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>
Message-Id: <20190117193658.16413-16-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>