Adding root xml tag name (VAST)

[milos-pesic-zattoo]

It would be great to include root xml tag name (VAST) to be able to catch problems with parsing other xml based formats (e.g vmap) earlier. Sometimes ad servers might respond with e.g vmap (xml format as well) in which case unmarshalling succeeds - and would be nicer to get an early error in that case.
It's one liner inside VAST struct:

XMLName xml.Name `xml:"VAST"`