The structs for individual device types need to be usable both as part
of the domain device list, and as top level XML documents on their own.
The latter is needed for device hotplug APIs. Thus all the devices must
have a XMLName tag present, and provide Marshal/Unmarshal methods. In
doing this, define a "Document" interface which declares the Marhsal/
Unmarhsal method contracts.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>