virbuf: improve testsuite reporting

virbuf: improve testsuite reporting

I had some temporary test failures while working on virbuf
improvements in later patches, with output that looked like:

Expected [<]
Actual [  <]

which is pretty hard to figure out.  Adding an Offset designation
made it much easier to find which particular '<' was at the
wrong indentation, to fix the right part of the code.

* tests/testutils.c (virtTestDifference): Make it easier to
diagnose test failures.