virtio console: don't manually set or finalize VIRTIO_CONSOLE_F_MULTIPORT.

That's already been done by the virtio infrastructure before the probe
function is called.

Reported-by: alexey.kardashevskiy@au1.ibm.com
Acked-by: Amit Shah <amit.shah@redhat.com>
Tested-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>