spapr, xics, xive: Move SpaprIrq::reset hook logic into activate/deactivate

spapr, xics, xive: Move SpaprIrq::reset hook logic into activate/deactivate

It turns out that all the logic in the SpaprIrq::reset hooks (and some in
the SpaprIrq::post_load hooks) isn't really related to resetting the irq
backend (that's handled by the backends' own reset routines).  Rather its
about getting the backend ready to be the active interrupt controller or
stopping being the active interrupt controller - reset (and post_load) is
just the only time that changes at present.

To make this flow clearer, move the logic into the explicit backend
activate and deactivate hooks.

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>