KVM: selftests: Use generic sys_clocksource_is_tsc() in vmx_nested_tsc_scaling_test

Despite its name, system_has_stable_tsc() just checks that system
clocksource is 'tsc'; this can now be done with generic
sys_clocksource_is_tsc().

No functional change intended.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20240109141121.1619463-3-vkuznets@redhat.com


Signed-off-by: Sean Christopherson <seanjc@google.com>