Option "split" does not available for kernel_irqchip flag in qemu-system-x86_64

On releases prior to Yakkety, the "split" option is not available for kernel_irqchip flag in qemu-system-x86_64.

Yakkety:
kernel_irqchip=on|off|split controls accelerated irqchip support (default=off)


Xenial:
kernel_irqchip=on|off controls accelerated irqchip support

Trusty:
kernel_irqchip=on|off controls accelerated irqchip support

Precise:
kernel_irqchip=on|off controls accelerated irqchip support

It will be great to have this option, as we will need this for some kvm-unit-tests for SRU