Avoid using 192.168.0.0/16

When using 192.168.0.0/16 address space on a 192.168.x.0/24 NAT'd
network (such as most home routers) VirtualBox will complain that the
virtual private network overlaps with an existing network and refuses to
start the VMs. 10.255.0.0/16 is a less-likely to conflict range, though
not perfect.