KVM: arm64: Key use of VHE instructions in nVHE code off ARM64_KVM_HVHE

KVM: arm64: Key use of VHE instructions in nVHE code off ARM64_KVM_HVHE

We can now start with the fun stuff: if we enable VHE *only* for
the hypervisor, we need to generate the VHE instructions when
accessing the system registers.

For this, reporpose the alternative sequence to be keyed off
ARM64_KVM_HVHE in the nVHE hypervisor code, and only there.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230609162200.2024064-10-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>