KVM: MMU: Avoid dropping accessed bit while removing write access

One more "KVM: MMU: Don't drop accessed bit while updating an spte."

Sptes are accessed by both kvm and hardware.
This patch uses update_spte() to fix the way of removing write access.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Avi Kivity <avi@redhat.com>