Adjust i386 Xen PV support for updated Xen interface files.

sys/i386/include/xen/xenvar.h:
sys/i386/xen/xen_machdep.c:
sys/xen/interface/foreign/structs.py:
sys/xen/evtchn/evtchn.c:
	MAX_VIRT_CPUS => XEN_LEGACY_MAX_VCPUS

Submitted by:	Roger Pau Monné
Reviewed by:	gibbs