KVM: arm64: Prevent NV feature flag on systems w/o nested virt

It would appear that userspace can select the NV feature flag regardless
of whether the system actually supports the feature. Obviously a nested
guest isn't getting far in this situation; let's reject the flag
instead.

Link: https://lore.kernel.org/r/20230920195036.1169791-6-oliver.upton@linux.devSigned-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kvm/arm.c

@@ -1208,6 +1208,9 @@ static unsigned long system_supported_vcpu_features(void)
 		clear_bit(KVM_ARM_VCPU_PTRAUTH_GENERIC, &features);
 	}
 
+	if (!cpus_have_final_cap(ARM64_HAS_NESTED_VIRT))
+		clear_bit(KVM_ARM_VCPU_HAS_EL2, &features);
+
 	return features;
 }