Add Gluster protocol as supported network disk backend

Add Gluster protocol as supported network disk backend

This patch introduces the RNG schema and updates necessary data strucutures
to allow various hypervisors to make use of Gluster protocol as one of the
supported network disk backend. Next patch will add support to make use of
this feature in Qemu since it now supports Gluster protocol as one of the
network based storage backend.

Two new optional attributes for <host> element are introduced - 'transport'
and 'socket'. Valid transport values are tcp, unix or rdma. If none specified,
tcp is assumed. If transport is unix, socket specifies path to unix socket.

This patch allows users to specify disks on gluster backends like this:

    <disk type='network' device='disk'>
      <driver name='qemu' type='raw'/>
      <source protocol='gluster' name='Volume1/image'>
        <host name='example.org' port='6000' transport='tcp'/>
      </source>
      <target dev='vda' bus='virtio'/>
    </disk>

    <disk type='network' device='disk'>
      <driver name='qemu' type='raw'/>
      <source protocol='gluster' name='Volume2/image'>
        <host transport='unix' socket='/path/to/sock'/>
      </source>
      <target dev='vdb' bus='virtio'/>
    </disk>

Signed-off-by: Harsh Prateek Bora <harsh@linux.vnet.ibm.com>