virttest.env_process: Process all images at once

Hi guys,

The first b679a97 patch is an important bugfix of cgroup_utils imports sent separately in https://github.com/autotest/virt-test/pull/612

The second d6bd3d2 patch changes the behavior of how images are checked. Previously VM was paused and resumed for each image. This cases really huge delays for multi_disk tests. With this patch the VM is paused only once and all images are then processed. Additionally failures are not raised immediately, but they are stored and all images are processed. This could help to delete all images and get all errors on failure.

The third 068dfcc patch fixes the problem of hanging screendump thread on postprocess failure.

Kind regards,
Lukáš