snapshot: simplify indentation of nwfilter

snapshot: simplify indentation of nwfilter

Fixing this involved some refactoring of common code out of
domain_conf and nwfilter_conf into nwfilter_params.

* src/conf/nwfilter_params.h (virNWFilterFormatParamAttributes):
Adjust signature.
* src/conf/nwfilter_params.c (_formatParameterAttrs)
(virNWFilterFormatParamAttributes): Adjust indentation handling,
and handle filterref here.
(formatterParam): Delete unused struct.
* src/conf/domain_conf.c (virDomainNetDefFormat): Adjust caller.
* src/conf/nwfilter_conf.c (virNWFilterIncludeDefFormat): Likewise.