xen: changes to hvm code in order to support PVH guests

On PVH we don't need to init the shared info page, or disable emulated
devices. Also, make sure PV IPIs are set before starting the APs.

Approved by: gibbs
Sponsored by: Citrix Systems R&D

x86/xen/hvm.c:

• Return early from functions that are no-ops on Xen PVH guests.
• In order to make sure PV IPIs are setup before AP startup, initialize them in SI_SUB_SMP-1.