x86/xen: use x{2}APIC if virtualized by hardware

Instead of using event channels or hypercalls to deal with IPIs and
NMIs.

Using a hardware virtualized APIC should be faster than using any PV
interface, since the VM exit can be avoided.

Xen exposes whether the domain is using hardware assisted x{2}APIC
emulation in a CPUID bit.

Sponsored by: Citrix Systems R&D