xen/intr: split xen_intr_isrc_lock uses

There are actually several distinct locking domains in xen_intr.c, but
all were sharing the same lock. Both xen_intr_port_to_isrc[] and the
x86 interrupt structures needed protection. Split these two apart as a
precursor to splitting the architecture portions off the file.

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