libvirt: Make sure NUMA cell memory is in Kb in XML

This patch makes libvirt report memory in Mb (as
hardware.VirtNUMATopologyUsage class expects Mb) and that when
generating XML we use Kb as this is what libvirt expects by default.

Change-Id: Ic4518acf6bcee463009437829646de8c83aff6bf
Closes-bug: #1373159
Merge "libvirt: Make sure NUMA cell memory is in Kb in XML"