spapr, xics, xive: Move cpu_intc_create from SpaprIrq to SpaprInterruptController

spapr, xics, xive: Move cpu_intc_create from SpaprIrq to SpaprInterruptController

This method essentially represents code which belongs to the interrupt
controller, but needs to be called on all possible intcs, rather than
just the currently active one.  The "dual" version therefore calls
into the xics and xive versions confusingly.

Handle this more directly, by making it instead a method on the intc
backend, and always calling it on every backend that exists.

While we're there, streamline the error reporting a bit.

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>