x86/altp2m: basic data structures and support routines

x86/altp2m: basic data structures and support routines

Add the basic data structures needed to support alternate p2m's and
the functions to initialise them and tear them down.

Although Intel hardware can handle 512 EPTP's per hardware thread
concurrently, only 10 per domain are supported in this patch for
performance reasons.

This change also splits the p2m lock into one lock type for altp2m's
and another type for all other p2m's. The purpose of this is to place
the altp2m list lock between the types, so the list lock can be
acquired whilst holding the host p2m lock.

Signed-off-by: Ed White <edmund.h.white@intel.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>
Cosmetic adjustments.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>