virtio: reset function

A reset function solves three problems:

1) It allows us to renegotiate features, eg. if we want to upgrade a
   guest driver without rebooting the guest.

2) It gives us a clean way of shutting down virtqueues: after a reset,
   we know that the buffers won't be used by the host, and

3) It helps the guest recover from messed-up drivers.

So we remove the ->shutdown hook, and the only way we now remove
feature bits is via reset.

We leave it to the driver to do the reset before it deletes queues:
the balloon driver, for example, needs to chat to the host in its
remove function.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

include/linux/virtio.h

@@ -45,9 +45,6 @@ struct virtqueue
  *	vq: the struct virtqueue we're talking about.
  *	This returns "false" (and doesn't re-enable) if there are pending
  *	buffers in the queue, to avoid a race.
- * @shutdown: "unadd" all buffers.
- *	vq: the struct virtqueue we're talking about.
- *	Remove everything from the queue.
  *
  * Locking rules are straightforward: the driver is responsible for
  * locking.  No two operations may be invoked simultaneously.
@@ -67,8 +64,6 @@ struct virtqueue_ops {
 
 	void (*disable_cb)(struct virtqueue *vq);
 	bool (*enable_cb)(struct virtqueue *vq);
-
-	void (*shutdown)(struct virtqueue *vq);
 };
 
 /**