RNG: Allow multiple parameters to be passed to an interface filter
    
    Our code supports that for ages. When using a <filterref/> to an
    <interface/> several parameters can be passed to the filter. Later,
    when building firewall rules, parameters are substituted for their
    values. However, our RNG schema allowed only one parameter to be
    passed.
    
    Reported-by: Brian Rak <brak@gameservers.com>
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>