virtio_config: Add virtio_config_val_len()

This patch adds virtio_config_val_len() which allows retrieving variable
length data from the virtio config space only if a specific feature is on.

Cc: Amit Shah <amit.shah@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: virtualization@lists.linux-foundation.org
Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

include/linux/virtio_config.h

@@ -155,6 +155,9 @@ static inline bool virtio_has_feature(const struct virtio_device *vdev,
 #define virtio_config_val(vdev, fbit, offset, v) \
 	virtio_config_buf((vdev), (fbit), (offset), (v), sizeof(*v))
 
+#define virtio_config_val_len(vdev, fbit, offset, v, len) \
+	virtio_config_buf((vdev), (fbit), (offset), (v), (len))
+
 static inline int virtio_config_buf(struct virtio_device *vdev,
 				    unsigned int fbit,
 				    unsigned int offset,