Move qemu_irq typedef out of qemu-common.h

It's necessary for making CPU child of DEVICE without
causing circular header deps.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
[ehabkost: re-added the typedef to hw/irq.h after rebasing]
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>