doc: add vm power mgmt overview svg

Added first of the two figures in the VM power management app UG:
VM power mangament highlevel overview

Signed-off-by: Alan Carew <alan.carew@intel.com>
Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Bernard Iremonger <bernard.iremonger@intel.com>