travis: add gcovr summary for GCOV build

travis: add gcovr summary for GCOV build

This gives a more useful summary, sorted by descending % coverage,
after the tests have run. The final numbers will give an idea if our
coverage is getting better or worse.

To keep the width sane we need to post process the file that the old
gcovr tool generates. This is done with a mix of sed, awk and column
in the scripts/coverage-summary.sh script.

As quite a lot of lines don't get covered at all we filter out all the
0% lines. If the file doesn't appear it is not being exercised.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>