backend: Check for shared storage also

If normal file storage was disabled but shared storage enabled,
“_TransformFileStorageDir” would still throw an exception.

in “opcodes._CheckStorageType” there's also a check, but I wasn't quite
sure what the correct way of handling it was, so I added a TODO comment.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>