qemu: cgroup: Don't use NULL path on default backed RNGs

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