xen/intr: move sys/x86/xen/xen_intr.c to sys/dev/xen/bus/

The event channel source code or equivalent is needed on all
architectures. Since much of this is viable to share, get this moved out
of x86-land. Each interrupt interface then needs a distinct back-end
implementation.

Reviewed by: royger
Submitted by: Elliott Mitchell <ehem+freebsd@m5p.com>
Original implementation: Julien Grall <julien@xen.org>, 2014-01-13 17:41:04
Differential Revision: https://reviews.freebsd.org/D30236