IPv6 will use prefix exclusively, and IPv4 will also optionally be
able to use it, and the iptables functions really need a prefix
anyway, so use the new virNetworkDefPrefix() function to send prefixes
into iptables functions instead of netmasks.

Also, in a couple places where a netmask is actually needed, use the
new private API function for it rather than getting it directly. This
will allow for cases where no netmask or prefix is specified (it
returns the default for the current class of network.)