x86/mm: Rework stale p2m auditing

x86/mm: Rework stale p2m auditing

The p2m audit code doesn't even compile, let alone work. It also
partially supports ept. Make it:

- compile
- lay groundwork for eventual ept support
- move out of the way of all calls and turn it into a domctl. It's
  obviously not being used by anybody presently.
- enable it via said domctl

Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Signed-off-by: Tim Deegan <tim@xen.org>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>