xen: remove xenpv_set_ids

The vcpu_id for legacy PVH mode can be set from the output of cpuid,
so there's no need to have a special function to set it.

Also note that xenpv_set_ids should have been executed only for PV
guests, but was executed for all guests types and vcpu_id was later
fixed up for HVM guests.

Reported by: cperciva
Approved by: re (gjb)
Sponsored by: Citrix Systems R&D