vsock/virtio: reset connected sockets on device removal

When the virtio transport device disappear, we should reset all
connected sockets in order to inform the users.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>