xen: Cleanup of mem_event structures

Signed-off-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Signed-off-by: Tamas K Lengyel <tklengyel@sec.in.tum.de>
xen: Rename mem_event to vm_event.

Signed-off-by: Tamas K Lengyel <tklengyel@sec.in.tum.de>