Fix error while opening rrd.file

[dpino]

There's an error while starting lwaftr run:

$ sudo ./snabb lwaftr run --cpu 10,11 --name lwaftr --conf lwaftr.conf --on-a-stick 82:00.0
Binding data-plane PID 18274 to CPU 10.
Bound main process to NUMA node:        1
Binding data-plane PID 18276 to CPU 11.
Error while running fiber: core/main.lua:26: File exists
Error while running fiber: core/main.lua:26: File exists
Error while running fiber: lib/ptree/ptree.lua:383: attempt to index a nil value

I think the problem is that rrd counters are created in the same folder as counters. When rrd counters are created int the pci folder, since the folder is now shared a process might to create a rrd counter that already exists.

[dpino]

On the other hand, there's an issue now with the snabb top view. Workers are not nested within their parent. Example: