hw/9118.c: Implement active-low interrupt support

hw/9118.c: Implement active-low interrupt support

The 9118 ethernet controller interrupt line is active low unless
the IRQ config register is programmed to set both the IRQ_POL
(polarity: active-high) and IRQ_TYPE (type: push-pull) bits:
implement support for inverting the irq output in other configurations.
This also requires that we support setting the bits in the first
place, and that we correctly preserve them across software reset.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andrzej Zaborowski <andrew.zaborowski@intel.com>