KVM: arm/arm64: implement kvm_arch_intc_initialized

On arm/arm64 the VGIC is dynamically instantiated and it is useful
to expose its state, especially for irqfd setup.

This patch defines __KVM_HAVE_ARCH_INTC_INITIALIZED and
implements kvm_arch_intc_initialized.

Signed-off-by: Eric Auger <eric.auger@linaro.org>
Acked-by: Christoffer Dall <christoffer.dall@linaro.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>