spapr: allocate the interrupt thread context under the CPU core

spapr: allocate the interrupt thread context under the CPU core

Each interrupt mode has its own specific interrupt presenter object,
that we store under the CPU object, one for XICS and one for XIVE.

Extend the sPAPR IRQ backend with a new handler to support them both.

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>