spapr/xive: Use the xics flag to check for XIVE-only IRQ backends

spapr/xive: Use the xics flag to check for XIVE-only IRQ backends

The sPAPR machine has four different IRQ backends, each implementing
the XICS or XIVE interrupt mode or both in the case of the 'dual'
backend.

If a machine is started in P8 compat mode, QEMU should necessarily
support the XICS interrupt mode and in that case, the XIVE-only IRQ
backend is invalid. Currently, spapr_irq_check() tests the pointer
value to the IRQ backend to check for this condition, instead use the
'xics' flag. It's equivalent and it will ease the introduction of new
XIVE-only IRQ backends if needed.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20200820140106.2357228-1-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>