Fix dummyvirsh.define() to return CmdResultObject.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
Fix the context_pattern in get_context_from_str method.

The old pattern will not match the '_' in context such as
'svirt_image_t'.

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