firewall: T4655: implement XML defaultValue for name and ipv6-name

This extends the implementation of commit 0cc7e0a49094 ("firewall: T4655: Fix
default action 'drop' for the firewall") in a way that we can now also use the
XML <defaultValue> node under "firewall name" and "firewall ipv6-name". This
is a much cleaner approach which also adds the default value automatically to
the CLIs completion helper ("?").