Add support for gluster volumes in libvirt

I added support for gluster disk on Libvirt.
Gluster disk doesn't have file attribute but does have name.
In glusterfs name are like 'gluster_volume_name/file'
See example:
    <disk type='network' device='disk'>
      <driver name='qemu' type='raw' cache='none'/>
      <source protocol='gluster' name='gv0/testglustervm.example.com-disk1'>
        <host name='127.0.0.1'/>
      </source>
      <target dev='vda' bus='virtio'/>
      <alias name='virtio-disk0'/>
    </disk>


See http://libvirt.org/formatdomain.html#elementsDisks