xen/xen-os: move inclusion of machine/xen-os.h later

Several of x86 enable/disable functions depend upon the xen*domain()
functions. As such the xen*domain() functions need to be declared
before machine/xen-os.h.

Officially declare direct inclusion of machine/xen/xen-os.h verboten as
such will break these functions/macros. Remove one such soon to be
broken inclusion.

Reviewed by: royger
Differential Revision: https://reviews.freebsd.org/D29811