virtio_balloon: add legacy_only flag

We have no plans to support virtio 1.0 in balloon driver.  Add an
explicit flag to mark it legacy only.

This will be used by follow-up patches.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_balloon.c

@@ -518,6 +518,7 @@ static unsigned int features[] = {
 };
 
 static struct virtio_driver virtio_balloon_driver = {
+	.legacy_only = true,
 	.feature_table = features,
 	.feature_table_size = ARRAY_SIZE(features),
 	.driver.name =	KBUILD_MODNAME,

include/linux/virtio.h

@@ -132,6 +132,7 @@ int virtio_device_restore(struct virtio_device *dev);
  * @feature_table_size: number of entries in the feature table array.
  * @feature_table_legacy: same as feature_table but when working in legacy mode.
  * @feature_table_size_legacy: number of entries in feature table legacy array.
+ * @legacy_only: driver does not support virtio 1.0.
  * @probe: the function to call when a device is found.  Returns 0 or -errno.
  * @remove: the function to call when a device is removed.
  * @config_changed: optional function to call when the device configuration
@@ -144,6 +145,7 @@ struct virtio_driver {
 	unsigned int feature_table_size;
 	const unsigned int *feature_table_legacy;
 	unsigned int feature_table_size_legacy;
+	bool legacy_only;
 	int (*probe)(struct virtio_device *dev);
 	void (*scan)(struct virtio_device *dev);
 	void (*remove)(struct virtio_device *dev);