The file detection logic for the command-line tool never detects XML and always interprets it as QTVR:
state 3: downloading testcases/good/spiv/mini.xml
state 3: file testcases/good/spiv/mini.xml downloaded
ReadMovieAtom: fread() failed!
state 7: Error during QTVR parsing: No panorama found
I'm not great with C++, but my interpretation of
src/libfreepv/Utils/signatures.cpp is that it will detect JPEG and PNG but then decide that everything else is QTVR.
The plugin doesn't do SPi-V either, though about:plugins lists it ok (I have no trouble with QTVR content):
application/x-director Schockwave dcr Yes
"Error during QTVR parsing: No panorama found"