hw/intc/arm_gicv3_its: Implement ITS base class

hw/intc/arm_gicv3_its: Implement ITS base class

This is the basic skeleton for both KVM and software-emulated ITS.
Since we already prepare status structure, we also introduce complete
VMState description. But, because we currently have no migratable
implementations, we also set unmigratable flag.

Signed-off-by: Pavel Fedin <p.fedin@samsung.com>
Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1474616617-366-3-git-send-email-eric.auger@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>