libvirt: fix VMXML.define and VMXML.undefine.

Why we "Allow any exceptions to propigate up" in vmxml.define()
and vmxml.undefine()????If define or undefine failed and there is no
error raised, user will be mistaken I think.

This patch fix define and undefine method in VMXML class to raise a
error if define or undefine fail.

Fixing the comment of virsh.undefine by the way.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>