[ARM] vgic: HW-assisted GICv2 emulation working on ARM Fast Models

This patch completes our HW-assisted GICv2 emulation (i.e. vGIC emulator).

We are able to boot Realview-PB-A8 linux guest with HW-assisted GICv2 on
VExpress-A15 emulated by ARM Fast Models.

More patches may follow as I try to optimize vGIC emulator and try to
run SMP Guest with vGIC.

Signed-off-by: Anup Patel <anup@brainfault.org>