Qemu once supported following memory stats which will returned by
"query_balloon":

    stat_put(dict, "actual", actual);
    stat_put(dict, "mem_swapped_in", dev->stats[VIRTIO_BALLOON_S_SWAP_IN]);
    stat_put(dict, "mem_swapped_out", dev->stats[VIRTIO_BALLOON_S_SWAP_OUT]);
    stat_put(dict, "major_page_faults", dev->stats[VIRTIO_BALLOON_S_MAJFLT]);
    stat_put(dict, "minor_page_faults", dev->stats[VIRTIO_BALLOON_S_MINFLT]);
    stat_put(dict, "free_mem", dev->stats[VIRTIO_BALLOON_S_MEMFREE]);
    stat_put(dict, "total_mem", dev->stats[VIRTIO_BALLOON_S_MEMTOT]);

But it later disabled all the stats except "actual" by commit
07b0403dfc2b2ac179ae5b48105096cc2d03375a.

libvirt doesn't parse "actual", so user will always see a empty result
with "virsh dommemstat $domain". Even qemu haven't disabled the stats,
we should support parsing "actual".