cgroup.cgroup: Merge autotest and virttest cgroup_utils library

Hi guys,

from the discussion about cgroup_utils I created those pull requests, which should merge `cgroup_utils`/`utils_cgroup` libraries into single file.

As there are outstanding pull request about cgroup_utils I don't want this to be applied now, just please consider this change and when the previous pull requests are finished I'll update those pull request.

Kind regards,
Lukáš Doktor