vircgroupmock: add support to test cgroup v2

vircgroupmock: add support to test cgroup v2

We need to create the cgroup v2 sysfs the same way as we do for
cgroup v1.

This introduces new VIR_CGROUP_MOCK_MODE env variable which will
configure which cgroup mode each test requires.  There are three
different modes:

    - legacy: only cgroup v1 is available and it's the default mode
    - hybrid: both cgroup v1 and cgroup v2 are available and have some
        controllers
    - unified: only cgroup v2 is available

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>