ppc/xics: remove set_nr_servers() handler from XICSStateClass

ppc/xics: remove set_nr_servers() handler from XICSStateClass

Today, the ICP (Interrupt Controller Presenter) objects are created by
the 'nr_servers' property handler of the XICS object and a class
handler. They are realized in the XICS object realize routine.

Let's simplify the process by creating the ICP objects along with the
XICS object at the machine level.

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