Libvirt doesn't report used memory correctly

Background: KVM's memory allocation behaviour: only allocates memory when guest touch that part of memory - this is *different* from memory ballooning which hypervisor is aware of what's going on and have control.
'Used' memory, from nova scheduler point of view, is allocated memory for guests/instances, no matter the guests/instances use them or not.

Because of this behavior, currently how libvirt+KVM reports 'used' memory (https://github.com/openstack/nova/blob/master/nova/virt/libvirt/driver.py#L2038) usually is less than allocated memory for all guest. Thus scheduler would schedule based on false information, this can be a problem when guests become active (use much more memory).