x86/mm: introduce a p2m class

x86/mm: introduce a p2m class

Use the class to differentiate between host and nested p2m's, and
potentially other classes in the future.

Fix p2m class checks that implicitly assume nested and host are
the only two classes that will ever exist.

Signed-off-by: Ed White <edmund.h.white@intel.com>
Acked-by: Tim Deegan <tim@xen.org>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>