Standardize how xml namespaces are handled in xml backend

[svniemeijer]

For the xml backend there are currently two ways in which xml namespace references can get stored in the internal coda definition for the product: as a space separated combination of namespace and element name (as returned by expat) inside the real_name of a record field, or as a separate xmlns attribute.

We should standardize on one approach and make sure this is used consistently everywhere.