updated Xen howto

[Manuel Bouyer <bouyer%antioche.lip6.fr@localhost>]

Hi,
I've added some details (binary snapshot, powerd, linux domains) to the howto
at:
http://www.netbsd.org/Ports/xen/howto.html

Please let me know if you notice any problem with what is here.

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--