xen/xenusb: always include xen/xen-os.h rather than machine/xen/xen-os.h

Fix compilation since machine/xen/xen-os.h is requiring definition
existing in xen/xen-os.h.

In general machine/xen/xen-os.h should never be included

Submitted by: Elliott Mitchell <ehem+freebsd@m5p.com>
Reviewed by: royger
Differential revision: https://reviews.freebsd.org/D29043