Display results from test cases

Considering the fixes I have published so far, it's of little surprise that I've been looking into using virt-test in our organization for running tests on hypervisors/VMs. I have however stumbled upon a missing feature that is needed to fulfill the requirements I got: The ability to historically save numeric test results.

We would find this feature useful for e.g. keeping a plot of the boot times over the last N releases, or to detect any trends in decreasing throughput when running netperf.

This patch takes care to only add the extra text when the TC is actually returning some value from the run() method. Example output:
(1/1) type_specific.io-github-autotest-qemu.boot_time: PASS [result=14.47s] (409.78 s)
which can be compared to the output when an upper bound is set (i.e. the default):
(1/1) type_specific.io-github-autotest-qemu.boot_time: PASS (409.78 s)
I have used boot_time as an example, and will include a link in this merge request. In case you want to test it out, I included a fix that was needed to run with JeOS as a guest. See this pull request for more information: https://github.com/autotest/tp-qemu/pull/64

So, what do you all think? Would more organizations find it useful? Any opinions on the output format?

Thanks,
/Jonas