Fix libvirt_xml/vm_xml.

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.

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>
Add attach_disk and dettach_disk in VM class.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
Add test for svirt attach disk.

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