add network disk support

add network disk support

This patch adds network disk support to libvirt/QEMU.  The currently
supported protocols are nbd, rbd, and sheepdog.  The XML syntax is like
this:

    <disk type="network" device="disk">
      <driver name="qemu" type="raw" />
      <source protocol='rbd|sheepdog|nbd' name="...some image identifier...">
        <host name="mon1.example.org" port="6000">
        <host name="mon2.example.org" port="6000">
        <host name="mon3.example.org" port="6000">
      </source>
      <target dev="vda" bus="virtio" />
    </disk>

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>