xen: mask out PWT too

The hypervisor doesn't allow PCD or PWT to be set on guest ptes, so
make sure they're masked out.  Also, fix up some previous mispatching.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>