Fix xm shutdown/reboot for HVM domain of IA64

Fix xm shutdown/reboot for HVM domain of IA64

xc.domain_destroy_hook() is called twice when we execute
xm shutdown/reboot command to an HVM domain without PV drivers.
The first calling is from shutdown() in XendDomainInfo.py.
The second calling is from destroyDomain() in XendDomainInfo.py.
The first calling is not necessary, so this patch removes it.

A discussion about this patch is as follows.
 http://lists.xensource.com/archives/html/xen-ia64-devel/2008-01/msg00232.html

Signed-off-by: Masaki Kanno <kanno.masaki@jp.fujitsu.com>