virtio-scsi: Cleanup of I/Os that never started

There is still a small window that occurs when a cancel I/O affects
an asynchronous I/O operation that hasn't started.  In other words,
when the residual data length equals the expected data length.

Today, the routine virtio_scsi_command_complete fails because the
VirtIOSCSIReq pointer (from the hba_private field in SCSIRequest)
was cleared earlier when virtio_scsi_complete_req was called by
the virtio_scsi_request_cancelled routine.  As a result, the
virtio_scsi_command_complete routine needs to simply return when
it is processing a SCSIRequest block that was marked canceled.
Signed-off-by: NEric Farman <farman@linux.vnet.ibm.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/scsi/virtio-scsi.c

@@ -306,6 +306,10 @@ static void virtio_scsi_command_complete(SCSIRequest *r, uint32_t status,
     VirtIOSCSIReq *req = r->hba_private;
     uint32_t sense_len;
 
+    if (r->io_canceled) {
+        return;
+    }
+
     req->resp.cmd->response = VIRTIO_SCSI_S_OK;
     req->resp.cmd->status = status;
     if (req->resp.cmd->status == GOOD) {