zoneconf: Fix bug in NIC assignment; Change visibility of unused zones

Fix a bug that allows users to add multiple NICs to non-bridged zones.
This fix includes a new error message.

Unused zones are now invisible instead of grey.

langs/en/cgi-bin/en.pl

@@ -2941,6 +2941,7 @@
 'zoneconf val ppp assignment error' => 'The NIC used for RED in PPP mode cannot be accessed by any other zone.',
 'zoneconf val vlan amount assignment error' => 'A zone cannot have more than one VLAN assigned.',
 'zoneconf val vlan tag assignment error' => 'You cannot use the same VLAN tag more than once per NIC.',
+'zoneconf val zoneslave amount error' => 'A zone that is not in bridge mode can\'t have more than one NIC assigned',
 );
 
 #EOF