KVM: MMU: Remove user access when allowing kernel access to gpte.w=0 page

If cr0.wp=0, we have to allow the guest kernel access to a page with pte.w=0.
We do that by setting spte.w=1, since the host cr0.wp must remain set so the
host can write protect pages.  Once we allow write access, we must remove
user access otherwise we mistakenly allow the user to write the page.

Reviewed-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Avi Kivity <avi@redhat.com>