xen: add an apic_enumerator for PVH

On PVH there's no ACPI, so the CPU enumeration must be implemented
using Xen hypercalls.

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

x86/xen/pvcpu_enum.c:

• Enumerate avaiable vCPUs on PVH by using the VCPUOP_is_up hypercall.
• Set vcpu_id for PVH guests.

conf/files.amd64:

• Include the PV CPU enumerator in the XENHVM build.