You also have to have new enough qemu that reports stats without
blocking (qemu 0.14 or newer added stat reporting via qom-get, since the
older query-balloon interface blocks) and libvirt that knows how to read
stats via qom-get (libvirt 1.1.1 or newer)
As I've already mentioned I use QEMU 2.0.0 and Libvirt 1.2.5 with no luck :(