Add a new connect_irq_notifier notifier in the SysBusDeviceClass. This
notifier, if populated, is called after sysbus_connect_irq.

This mechanism is used to setup VFIO signaling once VFIO platform
devices get attached to their platform bus, on a machine init done
notifier.
Signed-off-by: NEric Auger <eric.auger@linaro.org>
Reviewed-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>
Tested-by: NVikram Sethi <vikrams@codeaurora.org>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NAlex Williamson <alex.williamson@redhat.com>