Make the shadow PTE population code clear. Later we will add huge gtt
support based on this.

v2:
  - rebase to latest code.
Signed-off-by: NChangbin Du <changbin.du@intel.com>
Reviewed-by: NZhi Wang <zhi.wang@intel.com>
Signed-off-by: NZhenyu Wang <zhenyuw@linux.intel.com>