x86/altp2m: alternate p2m memory events

x86/altp2m: alternate p2m memory events

Add a flag to indicate that a memory event occurred in an alternate p2m
and a field containing the p2m index. Allow any event response to switch
to a different alternate p2m using the same flag and field.

Modify p2m_mem_access_check() to handle alternate p2m's.

Signed-off-by: Ed White <edmund.h.white@intel.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com> for the x86 bits.
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Tamas K Lengyel <tlengyel@novetta.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>