vsock/virtio: initialize vdev->priv before using VQs

When we fill VQs with empty buffers and kick the host, it may send
an interrupt. `vdev->priv` must be initialized before this since it
is used in the virtqueue callbacks.

Fixes: 0deab087

 ("vsock/virtio: use RCU to avoid use-after-free on the_virtio_vsock")
Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>