Fix virsh.start to return CmdResult object

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

Adapt all virsh.start() to new @return.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
Fix the nodedev_xml.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
virttest: Fix utils_selinux module.

Fix the context_pattern in get_context_from_str method.
The old pattern will not match the '_' in context such as
'svirt_image_t'.

utils_selinux_unittest: replace assertIn with assertTrue.

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