xen/intr: rework xen_intr_resume() for in-place remapping

The prior implementation of xen_intr_resume() was wiping
xen_intr_port_to_isrc[] and then rebuilding from the x86 interrupt
table. Rework to instead wipe the channel numbers (->xi_port) and then
scan the table for sources with invalid channels.

This will be slower due to scanning the whole table, but this removes
the dependency on the x86 interrupt code.

Reviewed by: royger
Differential Revision: https://reviews.freebsd.org/D30599
[royger]
Split line over 80 characters.