KVM: MMU: using kvm_set_pfn_accessed() instead of mark_page_accessed()

It's a small cleanup that using using kvm_set_pfn_accessed() instead
of mark_page_accessed()

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