x86/altp2m: add remaining support routines

x86/altp2m: add remaining support routines

Add the remaining routines required to support enabling the alternate
p2m functionality.

Signed-off-by: Ed White <edmund.h.white@intel.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>
Fix off-by-one in various checks against MAX_ALTP2M. Adjust error code
in p2m_destroy_altp2m_by_id(). Cosmetic adjustments.

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