scsi: fix cancellation when I/O was completed but DMA was not.

Commit d577646 (scsi: Introduce scsi_req_cancel_complete, 2014-09-25)
was supposed to have no semantic change, but it missed a case.  When
r->aiocb has already been NULLed, but DMA was not complete and the
SCSI layer was waiting for scsi_req_continue, after the patch the
SCSI layer will not call the .cancel callback of SCSIBusInfo.

Fixes: d5776465ee
Cc: qemu-stable@nongnu.org
Reported-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Tested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/scsi/scsi-bus.c

@@ -1770,6 +1770,8 @@ void scsi_req_cancel(SCSIRequest *req)
     req->io_canceled = true;
     if (req->aiocb) {
         blk_aio_cancel(req->aiocb);
+    } else {
+        scsi_req_cancel_complete(req);
     }
 }