virtio: console: Move code around for future patches

We're going to use add_port() from handle_control_message() in the next
patch.

Move the add_port() and fill_queue(), which depends on it, above
handle_control_message() to avoid forward declarations.

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