spapr: Clarify and fix handling of nr_irqs

spapr: Clarify and fix handling of nr_irqs

Both the XICS and XIVE interrupt backends have a "nr-irqs" property, but
it means slightly different things.  For XICS (or, strictly, the ICS) it
indicates the number of "real" external IRQs.  Those start at XICS_IRQ_BASE
(0x1000) and don't include the special IPI vector.  For XIVE, however, it
includes the whole IRQ space, including XIVE's many IPI vectors.

The spapr code currently doesn't handle this sensibly, with the
nr_irqs value in SpaprIrq having different meanings depending on the
backend.  We fix this by renaming nr_irqs to nr_xirqs and making it
always indicate just the number of external irqs, adjusting the value
we pass to XIVE accordingly.  We also move to using common constants
in most of the irq configurations, to make it clearer that the IRQ
space looks the same to the guest (and emulated devices), even if the
backend is different.

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>