add configuration for virsh undefine

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
modify define,undefine function

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
modify define/undefine usages in libvirt_vm

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
add test for virsh undefine

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
fix code style of virsh undefine.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
Add some illustrations for undefine configuration.

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