Good suggestion to create an XA datsource using the web interface (admin console).
After I created an XA datasource using the admin cvonsole, the differences were obvious.
An XA datasource (using an XA driver such as used in this example) should probably use the xa-datasource tag rather than the datasource tag - see the second code line in "Defining the Datasource itself". That choice of tag seems to govern which admin console tab displays the completed datasource.
Configuring an XA datasource using the standard datasource tag seems to work ok - my test servlet was able to lookup the datasource, get a connection, and execute an sql statement with no exceptions thrown.
If you choose to use xa-datasource, the datasource properties are defined using the xa-datasource-property tag.