Enable Hardware Assisted Paging per default

Libvirt should silently disable it if the CPU does not support it.
Requires second gen virtualization hardware:

* Intel i3/i5/i7 (Nehalem)
* AMD Opteron Barcelona

Supposed to give 40-100% performance increase, see
  http://en.wikipedia.org/w/index.php?title=Second_Level_Address_Translation&oldid=561955464