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

[ehem_freebsd_m5p.com]

Summary

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

Adapted-to-HEAD-by: Elliott Mitchell <[e-mail]>

[royger]

LGTM.

[ehem_freebsd_m5p.com]

This has the same issue of D29040, I had previously dealt with patch submission via mailing list and other review systems where the git author field was preserved, so crediting someone else you simply left them in the author field. Phabricator is the inverse where you need to add the original author as the footer, and the person submitting it for review gets the author field.

One issue, what are the conventional footer names? This particular one, this is simply a forward port where it is 99% the original author (simply a forward rebase). Some of this series needed rather substantial rework by me before submission (and perhaps only 75% original author's work). There are a few which were merely inspired by the original author.

I also wonder if there should be a "Group"/"Project"/"Umbrella" for Xen.