pseries: Refactor spapr irq allocation

pseries: Refactor spapr irq allocation

Paulo Bonzini changed the original spapr code, which manually assigned irq
numbers for each virtual device, to allocate them automatically from the
device initialization. That allowed spapr virtual devices to be constructed
with -device, which is a good start.  However, the way that patch worked
doesn't extend nicely for the future when we want to support devices other
than sPAPR VIO devices (e.g. virtio and PCI).

This patch rearranges the irq allocation to be global across the sPAPR
environment, so it can be used by other bus types as well.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>