virbuftest: Cleanup code around virTestDifference()

virbuftest: Cleanup code around virTestDifference()

Two things are happening here:

1) Call to virTestDifference() is guarded by '!result ||
   STRNEQ(result, _)' check. This is suboptimal since we have
   STRNEQ_NULLABLE().

2) There are couple of VIR_TEST_DEBUG() printings, which are
   useless. If debug is off they don't print anything, and if it
   is on, then much more information is printed by subsequent
   virTestDifference().

This makes the STRNEQ() + virTestDifference() combo look similar
to the rest of tests and thus can be picked up by spatch later.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Jonathon Jongsma <jjongsma@redhat.com>