virttest.env_process: Process all images at once

Hi guys,

The first patch is an important bugfix of cgroup_utils imports.

The second 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.

Kind regards,
Lukáš