libvirt-tests: Improve snapshot tests in libvirt.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
libvirt-snapshot: Improve virsh_snapshot test.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Update snapshot-delete function in virsh module.

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