LUInstanceCreate: fix file storage dir calculation

- Move the calculation at the beginning of CheckPrereq, since it doesn't
  modify any state, but still keeps locks
- Only perform the calculation if the actual disk template is filebased
- Error out if there is no defined file storage dir
- Only join the optional --file-storage-dir extra-path if one is passed

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>