virtio: drop internal struct from UAPI

There's no reason to expose struct vring_packed in UAPI - if we do we
won't be able to change or drop it, and it's not part of any interface.

Let's move it to virtio_ring.c

Cc: Tiwei Bie <tiwei.bie@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

include/uapi/linux/virtio_ring.h

@@ -213,14 +213,4 @@ struct vring_packed_desc {
 	__le16 flags;
 };
 
-struct vring_packed {
-	unsigned int num;
-
-	struct vring_packed_desc *desc;
-
-	struct vring_packed_desc_event *driver;
-
-	struct vring_packed_desc_event *device;
-};
-
 #endif /* _UAPI_LINUX_VIRTIO_RING_H */