Fix libvirt_xml/vm_xml.

This patch fix define and undefine method in VMXML class to return False
if define or undefine fail.

Fixing the comment of virsh.undefine by the way.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
libvirt.tests: Fix testcases to new XML define-undefine.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>