LinuxKPI: Add xen/xen.h header

[wulf]

Summary

It contains proxy-implementation of xen_initial_domain() and
xen_pv_domain() required by latest drm-kmod.

Sponsored by: Serenity Cyber Security, LLC
MFC after: 1 week

[ehem_freebsd_m5p.com]

sys/compat/linuxkpi/common/src/linux_compat.c

107–108

This is due to sys/amd64/include/xen/hypercall.h. I suspect the use is identical to what Linux headers used (the file originated with Linux as MIT/GPL2).

I think it would be reasonable to surround the #define __must_check in sys/amd64/include/xen/hypercall.h with #ifndef __must_check. This will interact with the merge from sys/amd64/include/xen/hypercall.h => sys/xen/hypercall.h though. @royger opinion on allowing use of the Linux-KPI's definition of __must_check?

[royger]

While the hypercall headers where originally imported from Linux, I think there's very little chance we will even sync again with Linuxes version, so s/__must_check/__result_use_check/ in the Xen hypercall headers would be the best solution IMO. Playing games with ifdef/ifndef is usually not a good idea.