virsh-schedinfo-qemu-posix: fix the cgroup path

Libvirt has changed the path where cgroups are stored and therefore
the tests fails for newer libvirt because it cannot get the value
of cgroups. To fix this, we should check if the old path exists and
eventualy change the path to new one and for sure also check it.

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