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.
The call structures around xen_intr_alloc_isrc() were rather perverse.
Notably the allocation function should be taking care of locking for
interrupt allocation needs and should also be handling the reuse of