libvirt_xml.devices: Remove re-assigned xml.

Slot 'xml' have been assigned in UntypedDeviceBase class.
So there is no need to set its value again in subclasses.
Otherwise, it will erase type_name in TypedDevice.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libvirt_xml.devices: Fix pep8 problems for disk module.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libvirt_xml.devices: Add method for address element in disk.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libvirt_xml.devices: Replace directly init with super for DiskSource.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
Merge pull request #992 from yumingfei/libvirt_xml_repairs

virttest.libvirt_xml: repairs for address and disk module.