Guests counter

Implemented Guest counter walking EPT (partial support for old "penryn" no-EPT processors too)
Removed EFER register check to detect IA-32e paging, it is not used by some hypervisor. Host address-space size from VM-exit controls is more reliable to this purpose.

Signed-off-by: Enrico Canzonieri <enrico.canzonieri@eurecom.fr>