virsh.schedinfo_qemu_posix: update the way we get the cgroup path

Now we have a new way how to obtain the correct cgroup path for
specific cgroup controller for a guest.

Let's use the new way also for the "schedinfo_qemu_posix" test.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Merge pull request #1167 from Antique/virsh-schedinfo-qemu-posix

virsh.schedinfo_qemu_posix: update the way we get the cgroup path