spapr: split the IRQ allocation sequence

spapr: split the IRQ allocation sequence

Today, when a device requests for IRQ number in a sPAPR machine, the
spapr_irq_alloc() routine first scans the ICSState status array to
find an empty slot and then performs the assignement of the selected
numbers. Split this sequence in two distinct routines : spapr_irq_find()
for lookups and spapr_irq_claim() for claiming the IRQ numbers.

This will ease the introduction of a static layout of IRQ numbers.

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