Avoid silent pass in case no cpuflags were detected for testing

In case no cpuflags were detected to be tested (after blacklisting) this
test silently passes not testing anything at all. Let's use
"test.cancel" to notify the user that nothing was actually executed.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

qemu/tests/cpuflags.py

@@ -1140,6 +1140,8 @@ def run(test, params, env):
         else:
             cpu_models = (set(get_cpu_models_supported_by_host()) -
                           set(cpu_model_black_list))
+            if not cpu_models:
+                test.cancel("No cpu_models detected, nothing to test.")
             logging.info("Start test with cpu models %s" % (str(cpu_models)))
             failed = []
             for cpumodel in cpu_models: