#274 Check network before creating persistent machine

[Beat Stebler]

If no network range is assigned to the pool and a persistent vm should be created, FOSS-Cloud remember me to do this. This is correct so far.

But the file on /var/virtualization/vm-persistent/<pool-id>/<vm-id>.qcow2 is created and can only be deleted by hand.</vm-id></pool-id>

The check, if the network range is assigned, have to be made before the copy of the image.