vhost-net: support VIRTIO_F_RING_RESET

Add VIRTIO_F_RING_RESET, which indicates that the driver can reset a
queue individually.

VIRTIO_F_RING_RESET feature is added to virtio-spec 1.2. The relevant
information is in
    oasis-tcs/virtio-spec#124
    oasis-tcs/virtio-spec#139

The implementation only adds the feature bit in supported features. It
does not require any other changes because we reuse the existing vhost
protocol.

The virtqueue reset process can be concluded as two parts:
1. The driver can reset a virtqueue. When it is triggered, we use the
set_backend to disable the virtqueue.
2. After the virtqueue is disabled, the driver may optionally re-enable
it. The process is basically similar to when the device is started,
except that the restart process does not need to set features and set
mem table since they do not change. QEMU will send messages containing
size, base, addr, kickfd and callfd of the virtqueue in order.
Specifically, the host kernel will receive these messages in order:
    a. VHOST_SET_VRING_NUM
    b. VHOST_SET_VRING_BASE
    c. VHOST_SET_VRING_ADDR
    d. VHOST_SET_VRING_KICK
    e. VHOST_SET_VRING_CALL
    f. VHOST_NET_SET_BACKEND
Finally, after we use set_backend to attach the virtqueue, the virtqueue
will be enabled and start to work.

Signed-off-by: Kangjie Xu <kangjie.xu@linux.alibaba.com>
Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Message-Id: <20220825085610.80315-1-kangjie.xu@linux.alibaba.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>

drivers/vhost/net.c

@@ -73,7 +73,8 @@ enum {
 	VHOST_NET_FEATURES = VHOST_FEATURES |
 			 (1ULL << VHOST_NET_F_VIRTIO_NET_HDR) |
 			 (1ULL << VIRTIO_NET_F_MRG_RXBUF) |
-			 (1ULL << VIRTIO_F_ACCESS_PLATFORM)
+			 (1ULL << VIRTIO_F_ACCESS_PLATFORM) |
+			 (1ULL << VIRTIO_F_RING_RESET)
 };
 
 enum {