Set up default /etc/network/interfaces with dhcp for eth0 in VM use case

Network interfaces in VM should listen to DHCP by default,
and if /etc/network/interfaces doesn't exist at all (so
takeover from host to guest isn't possible either) create
a sane default as well.

Thanks: Christian Hofstaedtler <christian@hofstaedtler.name> for suggestion + review
Thanks: Ulrich Dangel <mru@spamt.net> for review
Fixes #20