xen: fix mismerge in masking pte flags

Looks like a mismerge/misapply dropped one of the cases of pte flag
masking for Xen.  Also, only mask the flags for present ptes.

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