qemu.tests.virtio_console: Various fixups and new variants

Hi guys,

this took a bit longer as I found autotest infinite hang while improving the interrupted loopback test. The good news is that I localized the problem and fix all occurrences.

Please read the actual commit logs for details.
* 10bf772 - fix
* 1aaa213 - code cleanup and unification of Recv thread
* 7fcc603, 49cd84d, 11092dc - new variant of interrupted_loopback (current qemu dies on hotplug)
* 8313801 - fix infinite loop in Recv thread
* 42efd5e - prevent autotest hang caused by background threads
* 2d78d20 - styling changes
* 466af78 - fix the issue of current kernels which reports different message

Kind regards,
Lukáš