KVM: MMU: cleanup page alloc and free

Using __get_free_page instead of alloc_page and page_address,
using free_page instead of __free_page and virt_to_page

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