machine: Add comment to abort path in machine_set_kernel_irqchip

We're not supposed to abort when the user passes a bogus value.
Since the checking is done in visit_type_OnOffSplit(), the call
to abort() is legitimate. Let's add a comment to make it
explicit.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
