xen/intr: merge parts of resume functionality into new function

The portions of xen_rebind_ipi() and xen_rebind_virq() were already
near-identical. While xen_rebind_ipi() should panic() on
single-processor, still having the functionality to invoke seems
harmless.

Meanwhile much of the loop from xen_intr_resume() seemed to want to be
closer to this same code. This pushes related bits closer together.

Reviewed by: royger
Differential Revision: https://reviews.freebsd.org/D30598