virtio_console: fix uninitialized variable use

We try to disable callbacks on c_ivq even without multiport
even though that vq is not initialized in this configuration.

Fixes: c743d09d ("virtio: console: Disable callbacks for virtqueues at start of S4 freeze")
Suggested-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>