nwfilter: fix adding std MAC and IP values to filter binding

nwfilter: fix adding std MAC and IP values to filter binding

Commit d1a7c08eb changed filter instantiation code to ignore MAC and IP
variables explicitly specified for filter binding. It just replaces
explicit values with values associated with the binding. Before the
commit virNWFilterCreateVarsFrom was used so that explicit value
take precedence. Let's bring old behavior back.

This is useful. For example if domain has two interfaces it makes
sense to list both mac adresses in MAC var of every interface
filterref. So that if guest make a bond of these interfaces
and start sending frames with one of the mac adresses from
both interfaces we can pass outgress traffic from both
interfaces too.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>