cgroup: reduce complexity of controller disabling

cgroup: reduce complexity of controller disabling

This patch reduces the complexity of the filtering algorithm in
virCgroupDetect by first correcting the controller mask and then
checking for potential co-mounts without any correlating
controller mask modifications.

If you agree that this patch removes complexity and improves
readability it could simply be squashed into the first patch
of this series.

Signed-off-by: Boris Fiuczynski <fiuczy@linux.vnet.ibm.com>
Reviewed-by: Bjoern Walk <bwalk@linux.vnet.ibm.com>
Reviewed-by: Marc Hartmayer <mhartmay@linux.vnet.ibm.com>