arm64: KVM: Allow the main HYP code to use the init hyp stub implementation

We now have a full hyp-stub implementation in the KVM init code,
but the main KVM code only supports HVC_GET_VECTORS, which is not
enough.

Instead of reinventing the wheel, let's reuse the init implementation
by branching to the idmap page when called with a hyp-stub hypercall.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: James Morse <james.morse@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <cdall@linaro.org>