This moves quite a few of the x86-specific variables into a structure
separate from struct xenisrc, a first step in breaking them off.
Ideally these would never have left sys/x86/xen, but breaking them off at
the same time as moving the main portion of the file would have been
xi_activehi and xi_edgetrigger disappeared with the removal of PVHv1.
Originally they would have ended up in xen_arch_isrc, but now instead
Other architectures (ARM) assigns interrupt vectors via other means, thus
having the vector number in the general structure makes no sense.
This was inspired by the work of Julien Grall <firstname.lastname@example.org>,
2015-10-20 09:14:56, but merely borrows the "xen_arch_intr_t" name.