filter: make LibXSLTTransformer init backward compatible

As reported by Fernand Vanrie, initializing the XSLT service can fail
because the LibXSLTTransformer::initialize was changed to require
the parameters as a nested Sequence; accept previous parameter
convention as a fall-back.

(regression from ca0ea73a4ab104031a16b5bac7a9bb6e57c77ba0)

Change-Id: I136e6e6338f11ffecf9f856f0736d1d0e6b17c3f