[BUG] XML support for cpp-qt-qhttpengine-server generator

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

I'm currently using this tool to generate QHTTP server stubs and am having trouble getting it to generate xml code for models I have defined in my yaml file.

openapi-generator version

Pulled the latest docker image from docker hub

OpenAPI declaration file content or url

Attached the yaml code to this issue.

Generation Details

To generate the code, I run the following:
docker run --rm -v "${PWD}:/local" openapitools/openapi-generator-cli generate -i /local/ped.yaml -g cpp-qt-qhttpengine-server -o /local --additional-properties=withXml=true

Steps to reproduce

1. Generate the code
2. In the generated model files, you will only see "asJson" methods created

Related issues/PRs

Suggest a fix

Start by running the documented Docker generation command with the attached YAML and withXml=true. Inspect the generated model files and compare the available asJson methods with the expected XML support; done means the generated QHTTP server models include the intended XML handling.