Use the name of the chain rather than its type index (enum).
This pushes the later enablement of chains with user-given names
into the XML parser. For now we still only allow those names that
are well known ('root', 'arp', 'rarp', 'ipv4' and 'ipv6').
Signed-off-by: NStefan Berger <stefanb@linux.vnet.ibm.com>