TMF xml: Add toString() methods to the XML model classes

TMF xml: Add toString() methods to the XML model classes

And a message when throwing some exceptions so that we have more information
when debugging XML analyses.

Change-Id: I935f5727a612f9c4b96d5d8ade998ed4b673d3ba
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/33699
Tested-by: Hudson CI
Reviewed-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Tested-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>