qemu-nbd and nbd-client for qcow2 differential disk

Experiment: Same with a read only LV:
    # make new rw LV
    lvcreate -n testqemudiff -L 10G testvg

    # put some sample files there
    mkfs.ext4 -m 0 /dev/testvg/testqemudiff
    mount /dev/testvg/testqemudiff /mnt/test
    cp ~/archive/software/openSUSE/openSUSE-12.2-KDE-LiveCD-x86_64.iso /mnt/test
    umount /mnt/test

    # make it ro
    lvchange -p r testvg/testqemudiff

    # verify it is read only
    dd if=/dev/zero of=/dev/testvg/testqemudiff bs=1 count=1
        dd: failed to open ‘/dev/testvg/testqemudiff’: Read-only file system

    qemu-img create -b /dev/testvg/testqemudiff -f qcow2 testqemudiff.qcow2
        Formatting 'testqemudiff.qcow2', fmt=qcow2 size=10737418240 backing_file='/dev/testvg/testqemudiff' encryption=off cluster_size=65536 lazy_refcounts=off

    # try using the differential disk

    # start the nbd server (wrong way to do it for qcow2 ... this exports it raw)
        modprobe nbd
        zypper install nbd
        cp /etc/nbd-server/config{.example,}
        vim /etc/nbd-server/config
            [generic]
                # comment out the user and group
                # or create the user and group, and make sure the user can access the exported files

                listenaddr = 127.0.0.1
            [export1]
                exportname = /home/peter/tmp/qemu-base-disk/testqemudiff.qcow2
        /etc/init.d/nbd-server start

        # start the nbd client
        nbd-client localhost -N export1 /dev/nbd0

        # doesn't work ... this doesn't translate the qcow2 file into a raw

    # start using qemu-nbd (the right way to use qcow2)
    qemu-nbd -b 127.0.0.1 testqemudiff.qcow2
        (let it chill or nohup / bg it / whatever)
    nbd-client localhost 10809 /dev/nbd0