conf: clean up after adding calls to virNetDevIPInfo helpers
virDomainNetIPInfoParseXML() and virDomainNetIPInfoFormat() are no
longer "unused", so we can now remove the "ATTRIBUTE_UNUSED" from
their definitions, since virDomainNetIPInfoFormat() is now the only
caller of virDomainNetIPsFormat() and virDomainNetRoutesFormat(),
those two functions can simply be subsumed into
virDomainNetIPInfoFormat().