xen/intr: rework xen_intr_alloc_isrc() call structure

The call structure around xen_intr_alloc_isrc() was rather awful.
Notably finding a structure for reuse is part of allocation, but this
was done outside xen_intr_alloc_isrc(). Move this into
xen_intr_alloc_isrc() so the function handles all allocation steps.

Reviewed by: royger
Differential Revision: https://reviews.freebsd.org/D30726