vircgroup: add support for hybrid configuration
    
    This enables to use both cgroup v1 and v2 at the same time together
    with libvirt.  It is supported by kernel and there is valid use-case,
    not all controllers are implemented in cgroup v2 so there might be
    configurations where administrator would enable these missing
    controllers in cgroup v1.
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>