target-ppc: Initialize OpenPIC properly

When lowering an IRQ line, we search for the line we're supposed to lower.

Usually we run into an optimization there that queues up interrupts. This
queue ends with -1. Unfortunately we didn't set the first item to -1.

This patch fixes this, making interrupts work on PPC64.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
