test: Get "params" directly from "__param" in "get_state"

This should allow easier integration with children that override
"self.params". Such classes should simply use custom "self.params" and
report their "self.custom_params" while in "get_state" the stock Avocado
params would be used.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/core/test.py

@@ -645,7 +645,7 @@ class Test(unittest.TestCase, TestData):
         try:
             state['params'] = [(path, key, value)
                                for path, key, value
-                               in self.params.iteritems()]
+                               in self.__params.iteritems()]
         except Exception:
             state['params'] = None
         return state