The functions xen_intr_alloc_and_bind_ipi(), xen_register_pirq(),
xen_register_msi(), xen_release_msi() and xenpv_register_pirqs() are
specifically for x86-style interrupts. As sys/xen/xen_intr.h is in
common, it should be architecture-independent. Create
sys/x86/include/xen/xen_intr_x86.h for these x86-only functions.
Commit 27c36a12f1584b53d2454dac238eeed3dedc82ba is an x86-only feature.
As such the variable should only exist on x86.