virttest.qemu_devices: Some fixes and new unittest

Hi guys,

first 2 commits are fixtures we discussed in previous pull request. The rest is related to unittest and fixes found while developing it.

One thing I'm not sure about is eefde9d unittest. For future testing I need to add persistent outputs of qemu (help, devices, hmp, qmp). I thought about storing them in separate files, but didn't found a suitable place. For now I put them directly into the unittest, which looks ugly, but works fine. If you have a better solution, please let me know.

Kind regards,
Lukáš