virtio: add API to detect legacy devices

transports need to be able to detect legacy-only
devices (ATM balloon only) to use legacy path
to drive them.

Add a core API to do just that.
The implementation just blacklists balloon:
not too pretty, but let's not over-engineer.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>

drivers/virtio/virtio.c

@@ -3,6 +3,7 @@
 #include <linux/virtio_config.h>
 #include <linux/module.h>
 #include <linux/idr.h>
+#include <uapi/linux/virtio_ids.h>
 
 /* Unique numbering for virtio devices. */
 static DEFINE_IDA(virtio_index_ida);
@@ -267,6 +268,12 @@ static struct bus_type virtio_bus = {
 	.remove = virtio_dev_remove,
 };
 
+bool virtio_device_is_legacy_only(struct virtio_device_id id)
+{
+	return id.device == VIRTIO_ID_BALLOON;
+}
+EXPORT_SYMBOL_GPL(virtio_device_is_legacy_only);
+
 int register_virtio_driver(struct virtio_driver *driver)
 {
 	/* Catch this early. */