This reverts commit 0f80c718.

Turns out, our code relies on virCgroupFree(&var) setting
var = NULL.

Conflicts:
  src/util/vircgroup.c: context because 94f1855f is not
  reverted.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NPavel Hrdina <phrdina@redhat.com>