qemu: cgroup: Don't use NULL path on default backed RNGs
    
    The "random" backend for virtio-rng can be started with no path
    specified which equals to /dev/random. The cgroup code didn't consider
    this and called few of the functions with NULL resulting into:
    
     $ virsh start rng-vm
     error: Failed to start domain rng-vm
     error: Path '(null)' is not accessible: Bad address
    
    Problem introduced by commit c6320d34637a9883e31c4081d418fc33a4277cf2