virtio: console makes incorrect assumption about virtio API

The get_buf() API sets the second arg to the number of bytes *written*
by the other side; in this case it should be zero as these are output buffers.

lguest gets this right (obviously kvm's console doesn't), resulting in
continual buildup of console writes.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Amit Shah <amit.shah@redhat.com>