spapr/irq: introduce a spapr_irq_init_device() helper

The way the XICS and the XIVE devices are initialized follows the same
pattern. First, try to connect to the KVM device and if not possible
fallback on the emulated device, unless a kernel_irqchip is required.
The spapr_irq_init_device() routine implements this sequence in
generic way using new sPAPR IRQ handlers ->init_emu() and ->init_kvm().

The XIVE init sequence is moved under the associated sPAPR IRQ
->init() handler. This will change again when KVM support is added for
the dual interrupt mode.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20190513084245.25755-12-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>