virtio-console: avoid DMA from stack

put_chars() stuffs the buffer it gets into an sg, but that buffer may be
on the stack. This breaks with CONFIG_VMAP_STACK=y (for me, it
manifested as printks getting turned into NUL bytes).

Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
static void out_intr(struct virtqueue *vq)
{
	struct port *port;

	port = find_port_by_vq(vq->vdev->priv, vq);
	if (!port)
		return;

	wake_up_interruptible(&port->waitqueue);
}

static void out_intr(struct virtqueue *vq)
{
	struct port *port;

	port = find_port_by_vq(vq->vdev->priv, vq);
	if (!port)
		return;

	wake_up_interruptible(&port->waitqueue);
}
