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().