hw/arm/virt: add ITS support in virt GIC

Included creation of ITS as part of virt platform GIC
initialization. This Emulated ITS model now co-exists with kvm
ITS and is enabled in absence of kvm irq kernel support in a
platform.

Signed-off-by: Shashi Mallela <shashi.mallela@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20210910143951.92242-9-shashi.mallela@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>