libvirt-xml: Add methods to make xml take effect.

*Add undefine method to remove network.
*Add define method to define network from self.xml.
*Add start method to start network.
*Add sync method to call undefine and define.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
libvirt-tests: Add test for network bandwidth.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>