libvirt_xml_unittest: update unittest for libvirt_xml to adapt return-value of virsh.dumpxml.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
libvirt_xml: log out the CmdResult if base.validate() fails.

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