KVM: MMU: Fix is_dirty_pte()

is_dirty_pte() is used on guest ptes, not shadow ptes, so it needs to avoid
shadow_dirty_mask and use PT_DIRTY_MASK instead.

Misdetecting dirty pages could lead to unnecessarily setting the dirty bit
under EPT.

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