KVM: arm64: nv: Forward FEAT_XNX permissions to the shadow stage-2

KVM: arm64: nv: Forward FEAT_XNX permissions to the shadow stage-2

Add support for FEAT_XNX to shadow stage-2 MMUs, being careful to only
evaluate XN[0] when the feature is actually exposed to the VM.
Restructure the layering of permissions in the fault handler to assume
pX and uX then restricting based on the guest's stage-2 afterwards.

Reviewed-by: Marc Zyngier <maz@kernel.org>
Tested-by: Marc Zyngier <maz@kernel.org>
Link: https://msgid.link/20251124190158.177318-4-oupton@kernel.org
Signed-off-by: Oliver Upton <oupton@kernel.org>