xen/intr: fix corruption of event channel table

In xen_intr_release_isrc(), the isrc should only be removed if it is
assigned to a valid port. This had been mitigated by using 0 for not
having a port, but this is actually corrupting the table. Fix this bug
as modifying the code would cause this bug to manifest as kernel memory
corruption. Similar issue for the vCPU bitmap masks.

The KASSERT() doesn't need lock protection.

Reviewed by: royger
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30743