KVM: MMU: rename 'page' and 'shadow_page' to 'sp'

Rename 'page' and 'shadow_page' to 'sp' to better fit the context

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