KVM MMU: allow more shadow pages become asynchronous

Changlog v2:

- when level is PT_DIRECTORY_LEVEL, the 'offset' should be 'role.quadrant << 8', thanks Avi for point it out

- keep invlpg code in paging_tmpl.h address Avi's suggestion

- split kvm_sync_page() into kvm_sync_page() and kvm_sync_page_transient() to clarify the code address Avi's suggestion