spapr: change default interrupt mode to 'dual'

Now that XIVE support is complete (QEMU emulated and KVM devices),
change the pseries machine to advertise both interrupt modes: XICS
(P7/P8) and XIVE (P9).

The machine default interrupt modes depends on the version. Current
settings are:

    pseries   default interrupt mode

    4.1       dual
    4.0       xics
    3.1       xics
    3.0       legacy xics (different IRQ number space layout)

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20190522074016.10521-3-clg@kaod.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>