KVM: MMU: Don't treat a global pte as such if cr4.pge is cleared

The pte.g bit is meaningless if global pages are disabled; deferring
mmu page synchronization on these ptes will lead to the guest using stale
shadow ptes.

Fixes Vista x86 smp bootloader failure.

Signed-off-by: Avi Kivity <avi@redhat.com>