vircgroup: Introduce virCgroupControllerAvailable
    
    This new internal API checks if given CGroup controller is
    available.  It is going to be needed later when we need to make a
    decision whether pin domain memory onto NUMA nodes using cpuset
    CGroup controller or using numa_set_membind().
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>