xz: Change logging level for thread reduction to highest verbosity only.

xz: Change logging level for thread reduction to highest verbosity only.

Now that multi threaded encoding is the default, users do not need to
see a warning message everytime the number of threads is reduced. On
some machines, this could happen very often. It is not unreasonable for
users to need to set double verbose mode to see this kind of
information.

To see these warning messages -vv or --verbose --verbose must be passed
to set xz into the highest possible verbosity mode.

These warnings had caused automated testing frameworks to fail when they
expected no output to stderr.

Thanks to Sebastian Andrzej Siewior for reporting this and for the
initial version of the patch.