scsi: make code more homogeneous in AIO callback functions

First scsi_flush_complete, like scsi_dma_complete, is always called with
an active AIOCB.

Second, always test for "ret < 0" to check for errors.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/scsi-disk.c

@@ -160,7 +160,7 @@ static void scsi_dma_complete(void *opaque, int ret)
 
     bdrv_acct_done(s->qdev.conf.bs, &r->acct);
 
-    if (ret) {
+    if (ret < 0) {
         if (scsi_handle_rw_error(r, -ret)) {
             goto done;
         }
@@ -187,7 +187,7 @@ static void scsi_read_complete(void * opaque, int ret)
         bdrv_acct_done(s->qdev.conf.bs, &r->acct);
     }
 
-    if (ret) {
+    if (ret < 0) {
         if (scsi_handle_rw_error(r, -ret)) {
             goto done;
         }
@@ -211,10 +211,7 @@ static void scsi_flush_complete(void * opaque, int ret)
     SCSIDiskReq *r = (SCSIDiskReq *)opaque;
     SCSIDiskState *s = DO_UPCAST(SCSIDiskState, qdev, r->req.dev);
 
-    if (r->req.aiocb != NULL) {
-        r->req.aiocb = NULL;
     bdrv_acct_done(s->qdev.conf.bs, &r->acct);
-    }
 
     if (ret < 0) {
         if (scsi_handle_rw_error(r, -ret)) {
@@ -335,7 +332,7 @@ static void scsi_write_complete(void * opaque, int ret)
         bdrv_acct_done(s->qdev.conf.bs, &r->acct);
     }
 
-    if (ret) {
+    if (ret < 0) {
         if (scsi_handle_rw_error(r, -ret)) {
             goto done;
         }