RNG: Allow multiple parameters to be passed to an interface filter

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>
(cherry picked from commit 6cc5080a099a650c6cd98662219bfc5c9af153d7)