KVM: MMU: fix sync of ptes addressed at owner pagetable

During page sync, if a pagetable contains a self referencing pte (that
points to the pagetable), the corresponding spte may be marked as
writable even though all mappings are supposed to be write protected.

Fix by clearing page unsync before syncing individual sptes.

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