arm: Upgrade guest barriers to Outer-Shareable. Enable Protected Table Walk.

arm: Upgrade guest barriers to Outer-Shareable. Enable Protected Table Walk.

Upgrading barriers is conservative and may not be necessary.

Protected Table Walk traps stage 1 page tables which refer to device memory
(per stage 2) using a non-device mapping. This generally indicates a guest
error but trapping it as a fault for now helps us know if something odd is
going on.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Ian Campbell <ian.campbell@citrix.com>