KVM: PPC: Allow userspace to unset the IRQ line

Userspace can tell us that it wants to trigger an interrupt. But
so far it can't tell us that it wants to stop triggering one.

So let's interpret the parameter to the ioctl that we have anyways
to tell us if we want to raise or lower the interrupt line.

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

v2 -> v3:

 - Add CAP for unset irq
Signed-off-by: Avi Kivity <avi@redhat.com>