mem_access: support for memory-content hiding

mem_access: support for memory-content hiding

This patch adds support for memory-content hiding, by modifying the
value returned by emulated instructions that read certain memory
addresses that contain sensitive data. The patch only applies to
cases where VM_FLAG_ACCESS_EMULATE has been set to a vm_event
response.

Signed-off-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Acked-by: Tamas K Lengyel <tlengyel@novetta.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>