Using host-passthrough cpu mode with virtual hosts

Allowing to use all CPU feutures on the guest machins,
inclouding using nested guest feature when it is availible.

https://wiki.openstack.org/wiki/LibvirtXMLCPUModel
http://docs.openstack.org/developer/devstack/guides/devstack-with-nested-kvm.html

Change-Id: I32dd03f1f78192eef13e390e1c8547216e08a746

templates/domain.xml

@@ -2,6 +2,7 @@
   <name>%(name)s</name>
   <memory unit='KiB'>%(memory)s</memory>
   <vcpu>%(cpus)s</vcpu>
+  <cpu mode='host-passthrough' />
   <os>
     <type arch='%(arch)s'>hvm</type>
     <boot dev='%(bootdev)s'/>