virtio: allow drivers to validate features

Some drivers can't support all features in all configurations.  At the
moment we blindly set FEATURES_OK and later FAILED.  Support this better
by adding a callback drivers can use to do some early checks.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index 04b0d3f..7edfbdb 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -167,6 +167,7 @@ struct virtio_driver {
 	unsigned int feature_table_size;
 	const unsigned int *feature_table_legacy;
 	unsigned int feature_table_size_legacy;
+	int (*validate)(struct virtio_device *dev);
 	int (*probe)(struct virtio_device *dev);
 	void (*scan)(struct virtio_device *dev);
 	void (*remove)(struct virtio_device *dev);