virtio-rng: process pending requests on DRIVER_OK

virtio-rng device causes old guest kernels(2.6.32) to hang on latest qemu.
The driver attempts to read from the virtio-rng device too early in it's
initialization. Qemu detects guest is not ready and returns, resulting in
hang.

To fix handle pending requests when guest is running and driver status is
set to 'VIRTIO_CONFIG_S_DRIVER_OK'.

CC: qemu-stable@nongnu.org
Reported-by: Sergio lopez <slopezpa@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Pankaj Gupta <pagupta@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/virtio-rng.c

@@ -156,6 +156,19 @@ static void check_rate_limit(void *opaque)
     vrng->activate_timer = true;
 }
 
+static void virtio_rng_set_status(VirtIODevice *vdev, uint8_t status)
+{
+    VirtIORNG *vrng = VIRTIO_RNG(vdev);
+
+    if (!vdev->vm_running) {
+        return;
+    }
+    vdev->status = status;
+
+    /* Something changed, try to process buffers */
+    virtio_rng_process(vrng);
+}
+
 static void virtio_rng_device_realize(DeviceState *dev, Error **errp)
 {
     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
@@ -261,6 +274,7 @@ static void virtio_rng_class_init(ObjectClass *klass, void *data)
     vdc->realize = virtio_rng_device_realize;
     vdc->unrealize = virtio_rng_device_unrealize;
     vdc->get_features = get_features;
+    vdc->set_status = virtio_rng_set_status;
 }
 
 static const TypeInfo virtio_rng_info = {