KVM: arm/arm64: Guard kvm_vgic_map_is_active against !vgic_initialized

If the vgic is not initialized, don't try to grab its spinlocks or
traverse its data structures.

This is important because we soon have to start considering the active
state of a virtual interrupts when doing vcpu_load, which may happen
early on before the vgic is initialized.

Signed-off-by: Christoffer Dall <cdall@linaro.org>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>