Fix handling of disabled (shared) file storage

The vcluster changes broke the disabling of file storage; we can
workaround by (manually) skipping the virtualisation of file storage
paths if they are not enabled.

Note that tests/QA are still broken with disabled file storage; this
patch only fixes production code.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>