M: 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>
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