virtio-gpu: fix unmap in error path

We land here in case not everything we've asked for could be mapped.
So unmap only the bytes which have actually been mapped.

Also we didn't access anything, so acces_len can be 0.

Reported-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Message-id: 20190628072357.31782-1-kraxel@redhat.com

hw/display/virtio-gpu.c

@@ -1048,9 +1048,9 @@ static int virtio_gpu_load(QEMUFile *f, void *opaque, size_t size,
                 if (res->iov[i].iov_base) {
                     dma_memory_unmap(VIRTIO_DEVICE(g)->dma_as,
                                      res->iov[i].iov_base,
-                                     res->iov[i].iov_len,
+                                     len,
                                      DMA_DIRECTION_TO_DEVICE,
-                                     res->iov[i].iov_len);
+                                     0);
                 }
                 /* ...and the mappings for previous loop iterations */
                 res->iov_cnt = i;