[XEN] Fix early-unshadow detection for 3- or 4-level guest pagetables.

[XEN] Fix early-unshadow detection for 3- or 4-level guest pagetables.
Early-unshadow will unshadow whenever 2 zero values are written
to the same page; for PAE, one PTE takes 2 writes.  Only check
for the early unshadow when writing the low half of the PTE.