pc: initialize ioapic before use.

The changeset of 2c8d9340203c7f19265fd4cb2341f568217a3af6
prevents isa_irq_handler() from NULL refering of IsaIrqState::ioapic.
However it would be better to initialize the member before reference.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
