xen: fix PV IPI setup

So that it's done when the vcpu_id has been set. For the BSP the
vcpu_id is set at SUB_INTR, while for the APs it's done in
init_secondary_tail that's called at SUB_SMP order FIRST.

Reported and tested by: cperciva
Approved by: re (gjb)
Sponsored by: Citrix Systems R&D
Differential revision: https://reviews.freebsd.org/D17013