[xml] Alter --load-trace to show real paths of loaded files

[Alf Eaton <lists hubmed org>]

As highlighted in
<http://markmail.org/thread/xmupcsmm756tejaf#mid:limw5cwgeywqmwri>,
the output from xmllint when --load-trace is enabled shows the file
paths that would have been loaded if no catalog had been used, rather
than the *actual* files that were loaded.

Could this be changed, or another section added to the output that
shows which file was actually loaded? It would help greatly when
debugging catalogs.

alf