virtio: console: Handle port hot-plug

If the 'nr_ports' variable in the config space is updated to a higher
value, that means new ports have been hotplugged.

Introduce a new workqueue to handle such updates and create new ports.

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