We shouldn't call kvmclock_create() when KVM is not available
or disabled:
 - check for kvm_enabled() before calling it
 - assert KVM is enabled once called
Since the call is elided when KVM is not available, we can
remove the stub (it is never compiled).

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230620083228.88796-2-philmd@linaro.org>