Line and ports.list fix

- enable the use of line topologies. Without the fix, an error message like this would show up when one would try to start a line topology:
 
	Traceback (most recent call last):
	File "./vnet", line 458, in <module>
	main()
	File "./vnet", line 292, in main
	node.start()
	File "/home/lucas/riot/desvirt_mehlis/desvirt/vm.py", line 95, in start
	self.vm_instance.create()
	File "/home/lucas/riot/desvirt_mehlis/desvirt/riotnative.py", line 57, in create
	y = int(position[1]) #1..5
	IndexError: string index out of range

- create ports.list if it doesn't exist yet to aboid more nasty errors
- add state files for topology xmls, locks, macs, pids taps and the ports.list as well as ._* and .DS_Store to .gitignore.