scripts: add a XML preprocessor script

gdbus-codegen doesn't support conditions or pre-processing.

Rather than duplicating D-Bus interfaces for win32 adaptation, let's
have a preprocess step, so we can have platform-specific interfaces.

The python script is based on
https://github.com/peitaosu/XML-Preprocessor, with bug fixes, some
testing and replacing lxml dependency with the built-in xml module.

This preprocessing syntax style is not very common, but is similar to
the one provided by WiX (https://wixtoolset.org/docs/v3/overview/preprocessor/

)
or wixl, that we adopted in QEMU for packaging the guest agent.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20230606115658.677673-5-marcandre.lureau@redhat.com>