Re-implement as a link setter. This should allow the QOM framework to
keep track of ref counts properly etc.

We need to add a default parent for the connecting input incase it's
coming from a non-qdev source. We simply parent the IRQ to the machine
in this case.

Reviewed-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>