virttest.qemu_devices: Add hotplug support

Hi guys,

this pull request is on top of the https://github.com/autotest/virt-test/pull/666 one. I fixed some issues, but as I simultaneously worked on hotplug it would be hard to rebase everything. I'll continue on the problems addressed there, but if you're not strongly against it I'd rather fix it on top of this patchset to avoid complex rebases, when hotplug support was also something I promised to do.

regards,
Lukáš