virtio-bus: cleanup plug/unplug interface

Right now we have these pairs:

- virtio_bus_plug_device/virtio_bus_destroy_device.  The first
  takes a VirtIODevice, the second takes a VirtioBusState

- device_plugged/device_unplug callbacks in the VirtioBusClass
  (here it's just the naming that is inconsistent)

- virtio_bus_destroy_device is not called by anyone (and since
  it calls qdev_free, it would be called by the proxies---but
  then the callback is useless since the proxies can do whatever
  they want before calling virtio_bus_destroy_device)

And there is a k->init but no k->exit, hence virtio_device_exit is
overwritten by subclasses (except virtio-9p).  This cleans it up by:

- renaming the device_unplug callback to device_unplugged

- renaming virtio_bus_plug_device to virtio_bus_device_plugged,
  matching the callback name

- renaming virtio_bus_destroy_device to virtio_bus_device_unplugged,
  removing the qdev_free, making it take a VirtIODevice and calling it
  from virtio_device_exit

- adding a k->exit callback

virtio_device_exit is still overwritten, the next patches will fix that.

Cc: qemu-stable@nongnu.org
Acked-by: Andreas Faerber <afaerber@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/virtio/virtio.c

@@ -1158,14 +1158,19 @@ static int virtio_device_init(DeviceState *qdev)
     if (k->init(vdev) < 0) {
         return -1;
     }
-    virtio_bus_plug_device(vdev);
+    virtio_bus_device_plugged(vdev);
     return 0;
 }
 
 static int virtio_device_exit(DeviceState *qdev)
 {
     VirtIODevice *vdev = VIRTIO_DEVICE(qdev);
+    VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(qdev);
 
+    virtio_bus_device_unplugged(vdev);
+    if (k->exit) {
+        k->exit(vdev);
+    }
     if (vdev->bus_name) {
         g_free(vdev->bus_name);
         vdev->bus_name = NULL;