I'm not using xa-datasource, so your mileage may differ, but I provide that in the datasource definition via the <connection-url>jdbc:....</connection-url> node.
{code:xml}
<datasources>
<datasource jndi-name="java:/jboss/datasources/MyDS"
pool-name="MyDS"
enabled="true"
use-java-context="true">
<connection-url>@database.url@</connection-url>
<driver>@database.driver@</driver>
<pool>
<min-pool-size>100</min-pool-size>
<max-pool-size>200</max-pool-size>
</pool>
<security>
<user-name>@database.username@</user-name>
<password>@database.password@</password>
</security>
<statement>
<prepared-statement-cache-size>100</prepared-statement-cache-size>
<share-prepared-statements>true</share-prepared-statements>
</statement>
</datasource>
</datasources>
{code}
The @database.*@ lines obviously need to be replaced with your database setup.