Is there any reason to keep a free function?
It's internal for SCSIDevice implementation, kind of a "base
implementation" for free_req
The pattern should be
that people just call the function to decrement the reference count,
and that frees the structure when it hits zero. In the current model
that would mean moving the freeing out of ->free_req into scsi_req_unref,
but that seems pretty sensible anyway.
free_req is still needed, because it takes care of freeing the bounce
buffers or any other allocated data.