private ConfigInstance.Builder getRootBuilder() {
        ConfigInstance.Builder builder = null;
        Class<?>[] classes = clazz.getDeclaredClasses();
        for (Class<?> c : classes) {
            if (c.getName().endsWith("Builder")) {
                try {
                    builder = (ConfigInstance.Builder) c.getConstructor().newInstance();
                } catch (Exception e) {
                    throw new RuntimeException("Could not instantiate builder for " + clazz.getName(), e);
                }
            }
        }
        if (builder == null) {
            throw new RuntimeException("Could not find builder for " + clazz.getName());
        } else {
            return builder;
        }
    }
Create a ConfigBuilder from a payload, based on the <code>clazz</code> supplied.

@param payload a Payload to be transformed to builder
@return a ConfigBuilder
config/src/main/java/com/yahoo/vespa/config/ConfigTransformer.java