KVM: arm64: Allow userspace to limit NV support to nVHE

KVM: arm64: Allow userspace to limit NV support to nVHE

NV is hard. No kidding.

In order to make things simpler, we have established that NV would
support two mutually exclusive configurations:

- VHE-only, and supporting recursive virtualisation

- nVHE-only, and not supporting recursive virtualisation

For that purpose, introduce a new vcpu feature flag that denotes
the second configuration. We use this flag to limit the idregs
further.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Link: https://lore.kernel.org/r/20250220134907.554085-11-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>