Make PGD preloading mandatory and remove enable_pgtbl_preload parameter

[laijs]

Issues #14 and code #15 introduced excellent code to preload the user PGD's shadow pagetable, but it is currently controlled by the module parameter enable_pgtbl_preload.

The feature works well, and it would be better to enable it by default (make it mandatory) and remove the module parameter.

By making preloading mandatory, we can also simply other code that checks the hw_CR3s a lot.

cc @zhuangel