irq: introduce qemu_irq_proxy()

irq: introduce qemu_irq_proxy()

In some cases we have a circular dependency involving irqs - the irq
controller depends on a bus, which in turn depends on the irq controller.
Add qemu_irq_proxy() which acts as a passthrough, except that the target
irq may be set later on.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>