KVM: MMU: Add validate_direct_spte() helper

Add a helper to verify that a direct shadow page is valid wrt the required
access permissions; drop the page if it is not valid.

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