Disk: add new dataStore element to disk.py

[meinaLi]

Now a new element dataStore has been implemented. So add it to disk.py. I also found that the "attrs" has included the "dev" and "file" in "slots = ("attrs", "dev", "protocol", "name", "host", "file", "auth")" actually. So I've also updated it.

[meinaLi]

>>>from virttest.libvirt_xml.devices import disk
>>>disk_dev = disk.Disk()
>>>disk_dev.xml = """
<disk type='file' device='disk'>
    <driver name='qemu' type='qcow2'/>
    <source file='/path/to/datastore.qcow2'>
      <dataStore type='file'>
        <format type='raw'/>
        <source file='/path/to/datastore'/>
      </dataStore>
    </source>
    <backingStore type='file'>
      <format type='qcow2'/>
      <source file='/var/lib/libvirt/images/base-with-data-file.qcow'>
        <dataStore type='block'>
          <format type='raw'/>
          <source dev='/dev/mapper/base2'/>
        </dataStore>
      </source>
    </backingStore>
    <target dev='vdh' bus='virtio'/>
  </disk>
  """
>>>disk_dev.fetch_attrs()
{'target': {'dev': 'vdh', 'bus': 'virtio'},
 'source': {'attrs': {'file': '/path/to/datastore.qcow2'},
  'dataStore': {'source': {'attrs': {'file': '/path/to/datastore'}},
   'type': 'file',
   'format': {'type': 'raw'}}},
 'driver': {'name': 'qemu', 'type': 'qcow2'},
 'device': 'disk',
 'type_name': 'file',
 'backingstore': {'source': {'datastore': {'source': {'attrs': {'dev': '/dev/mapper/base2'}},
    'type': 'block',
    'format': {'type': 'raw'}},
   'attrs': {'file': '/var/lib/libvirt/images/base-with-data-file.qcow'}},
  'type': 'file',
  'format': {'type': 'qcow2'}}}