virtio: console: struct ports for multiple ports per device.

Rather than assume a single port, add a 'struct ports_device' which
stores data related to all the ports for that device.

Currently, there's only one port and is hooked up with hvc, but that
will change.

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