x86/mm: New mem access type to log access

x86/mm: New mem access type to log access

This patch adds a new p2m access type, n2rwx. It allows for implement a "log
access" mode in the hypervisor, akin to log dirty but for all types of
accesses. Faults caused by this access mode automatically promote the
access rights of the offending p2m entry, place the event in the ring, and
let the vcpu keep on executing.

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