Xen always runs on CPUs which properly support WP enforcement in
privileged mode, so there's no need to test for it.

This also works around a crash reported by Arnd Hannemann, though I
think its just a band-aid for that case.
Reported-by: NArnd Hannemann <hannemann@nets.rwth-aachen.de>
Signed-off-by: NJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Acked-by: NPekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>