virtio-blk: handle NULL returns from bdrv_aio_{read, write}

The bdrv_aio_{read,write} routines can return a NULL pointer when the
I/O submission fails.  Currently we ignore this and will wait forever
for an I/O completion and leading to a hang of the guest.

I can easily reproduce this using the native Linux AIO patch, but it's
also possible using normal pthreads-based AIO.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/virtio-blk.c

@@ -254,14 +254,24 @@ static void virtio_blk_handle_scsi(VirtIOBlockReq *req)
 
 static void virtio_blk_handle_write(VirtIOBlockReq *req)
 {
-    bdrv_aio_writev(req->dev->bs, req->out->sector, &req->qiov,
+    BlockDriverAIOCB *acb;
+
+    acb = bdrv_aio_writev(req->dev->bs, req->out->sector, &req->qiov,
                           req->qiov.size / 512, virtio_blk_rw_complete, req);
+    if (!acb) {
+        virtio_blk_req_complete(req, VIRTIO_BLK_S_IOERR);
+    }
 }
 
 static void virtio_blk_handle_read(VirtIOBlockReq *req)
 {
-    bdrv_aio_readv(req->dev->bs, req->out->sector, &req->qiov,
+    BlockDriverAIOCB *acb;
+
+    acb = bdrv_aio_readv(req->dev->bs, req->out->sector, &req->qiov,
                          req->qiov.size / 512, virtio_blk_rw_complete, req);
+    if (!acb) {
+        virtio_blk_req_complete(req, VIRTIO_BLK_S_IOERR);
+    }
 }
 
 static void virtio_blk_handle_output(VirtIODevice *vdev, VirtQueue *vq)