vboxrun.sh: Make sure there's a host-only network we can attach to

[alfreb]

Currently we just assume the user has a host-only network called vboxnet0 set up. We should check if such a network exists, or otherwise create one. See:

• https://www.virtualbox.org/manual/ch08.html#vboxmanage-hostonlyif

Also add DHCP. See:

• https://www.virtualbox.org/manual/ch08.html#vboxmanage-dhcpserver