revert: libvirt is happy with redefinition of an existing nwfilter

Change-Id: If7ab71951ae4edef750be9a4f944e3180d568c91
Bug-Url: https://bugzilla.redhat.com/1138807
Signed-off-by: Petr Horáček <phoracek@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/32691
Reviewed-by: Dan Kenigsberg <danken@redhat.com>