To reproduce give any invalid root id.

`openv-server --root /dev/riot/tty-iotlab-m3_`

```
15:15:42 WARNING Probing motes: [] at baudrates ['500000']
15:15:42 SUCCESS Discovered serial-port(s): []
15:15:42 INFO extracting firmware definitions.
15:15:42 VERBOSE extracting firmware component names
15:15:42 VERBOSE extracting firmware log descriptions.
15:15:42 VERBOSE extracting 6top return codes.
15:15:42 VERBOSE extracting 6top states.
15:15:42 INFO setting DAG root...
Traceback (most recent call last):
File "/home/francisco/.virtualenvs/openv/bin/openv-server", line 8, in <module>
sys.exit(main())
File "/home/francisco/workspace/openwsn/openvisualizer/openvisualizer/main.py", line 970, in main
iotlab_passwd=args.password,
File "/home/francisco/workspace/openwsn/openvisualizer/openvisualizer/main.py", line 285, in _init_
self.set_root(self.root)
File "/home/francisco/workspace/openwsn/openvisualizer/openvisualizer/main.py", line 523, in set_root
raise Fault(faultCode=-1, faultString="Unknown port or address: {}".format(port_or_address))
xmlrpclib.Fault: <Fault -1: 'Unknown port or address: /dev/riot/tty-iotlab-m3_'>
^C^CTerminated
```