netConf: fix a sloppy rebase

Commit 5cabaaa1 is refering to ConfigWriter.NET_CONF_PREF which was
dropped by a parallel commit 1ebff6e5b.

Change-Id: Ibecb8d6797f64e4239d7078566fbca5b89f030fc
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/10582
Reviewed-by: Igor Lvovsky <ilvovsky@redhat.com>