qemu.tests.multi_disk_random_hotplug: New test

Hi guys,

this pull request contains the promised hotplug test. Unfortunately I couldn't finish it properly, because there are higher priority tasks I need to work on. Anyway this version works, it only is not as nice as I'd like it to be.

First three patches are part of https://github.com/autotest/virt-test/pull/770 pull request.

The following thee ones are some few modifications and bug fixes I encounter while developing this test.

The last patch is the actual test. Currently it supports only disks hotplug and it still modifies the test params to work with qemu_qtree verification (and image removal).

The idea is to develop qemu_qtree vs. qemu_devices verification and allow to develop multi-purpose hotplug test. It should be doable, I played with it a bit, but currently I need to postpone this development.
Additionally the images should be created automatically by qemu_devices during VM.create() or during startup. This way we finally move away from issues which modification of params brings.

I believe this test is usable and can be applied. Anyway I'd like to work on it once things settle down a bit.