x86/mm: New sharing audit memop

x86/mm: New sharing audit memop

Remove costly mem_sharing audits from the inline path, and instead make them
callable as a memop.

Have the audit function return the number of errors detected.

Update memshrtool to be able to trigger audits.

Set sharing audits as enabled by default.

Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Signed-off-by: Adin Scannell <adin@scannell.ca>
Acked-by: Tim Deegan <tim@xen.org>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Tim Deegan <tim@xen.org>