OOXML embedded objects are converted to ODF during export, which
prevents the export from working, as it expects a stream, and ODF
uses a storage.

There is already a unit test in testEmbeddedXlsx, but it wrongly
succeeds because the components necessary to convert from OOXML to ODF
are missing in the test environment.

(cherry picked from commit 1dac99e7)

Change-Id: Ib4df346e04ecfb54ec1a728535be876db921b884