scsi: fix refcounting for reads

Recently introduced FUA support also gave us a use-after-free
of the BlockAcctCookie within a SCSIDiskReq, due to unbalanced
reference counting.

The patch fixes this by making scsi_do_read look like a combination
of scsi_*_complete + scsi_*_data.  It does both a ref (like
scsi_read_data) and an unref (like scsi_flush_complete).
Reported-by: NDavid Gibson <david@gibson.dropbear.id.au>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/scsi-disk.c

@@ -296,6 +296,13 @@ static void scsi_do_read(void *opaque, int ret)
         }
     }
 
+    if (r->req.io_canceled) {
+        return;
+    }
+
+    /* The request is used as the AIO opaque value, so add a ref.  */
+    scsi_req_ref(&r->req);
+
     if (r->req.sg) {
         dma_acct_start(s->qdev.conf.bs, &r->acct, r->req.sg, BDRV_ACCT_READ);
         r->req.resid -= r->req.sg->size;