virtio: disable multiport console support.

Move MULTIPORT feature and related config changes
out of exported headers, and disable the feature
at runtime.

At this point, it seems less risky to keep code around
until we can enable it than rip it out completely.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>