spapr, xics, xive: Move SpaprIrq::post_load hook to backends

spapr, xics, xive: Move SpaprIrq::post_load hook to backends

The remaining logic in the post_load hook really belongs to the interrupt
controller backends, and just needs to be called on the active controller
(after the active controller is set to the right thing based on the
incoming migration in the generic spapr_irq_post_load() logic).

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>