KVM: PPC: Implement level interrupts for BookE

BookE also wants to support level based interrupts, so let's implement
all the necessary logic there. We need to trick a bit here because the
irqprios are 1:1 assigned to architecture defined values. But since there
is some space left there, we can just pick a random one and move it later
on - it's internal anyways.

Signed-off-by: Alexander Graf <agraf@suse.de>