virtio: console: Use a control message to add ports

Instead of the host and guest independently enumerating ports, switch to
a control message to add ports where the host supplies the port number
so there's no ambiguity or a possibility of a race between the host and
the guest port numbers.

We now no longer need the 'nr_ports' config value. Since no kernel has
been released with the MULTIPORT changes yet, we have a chance to fiddle
with the config space without adding compatibility features.

This is beneficial for management software, which would now be able to
instantiate ports at known locations and avoid problems that arise with
implicit numbering in the host and the guest. This removes the 'guessing
game' part of it, and management software can now actually indicate
which id to spawn a particular port on.

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