xen: move x86/xen/xenpv.c to dev/xen/bus/xenpv.c

Minor changes are necessary to make this processor-independent, but
moving the file out of x86 and into common is the first step (so
others don't add /more/ x86-isms).

Submitted by: Elliott Mitchell <ehem+freebsd@m5p.com>
Reviewed by: royger
Differential Revision: https://reviews.freebsd.org/D29042