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