spapr: Move SpaprIrq::nr_xirqs to SpaprMachineClass

spapr: Move SpaprIrq::nr_xirqs to SpaprMachineClass

For the benefit of peripheral device allocation, the number of available
irqs really wants to be the same on a given machine type version,
regardless of what irq backends we are using.  That's the case now, but
only because we make sure the different SpaprIrq instances have the same
value except for the special legacy one.

Since this really only depends on machine type version, move the value to
SpaprMachineClass instead of SpaprIrq.  This also puts the code to set it
to the lower value on old machine types right next to setting
legacy_irq_allocation, which needs to go hand in hand.

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>