debnet: Ignore loop devices

debnet: Ignore loop devices

Quoting from http://bts.grml.org/grml/issue1166:

| I booted using the options lvm debnet, expecting debnet to use the
| /etc/network/interfaces file from the LVM logical volume. Instead, it
| used the /etc/network/interfaces file from /dev/loop0, because that is
| listed earlier in /proc/partitions.

Thanks: John S. Skogtvedt <jss@bzz.no> for bugreport and patch