spapr/irq: initialize the IRQ device only once

Add a check to make sure that the routine initializing the emulated
IRQ device is called once. We don't have much to test on the XICS
side, so we introduce a 'init' boolean under ICSState.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20190513084245.25755-13-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>