[2854] replaced expressions using xor with minux.

for some readers the latter seems to be more understandable

src/bin/memmgr/tests/memmgr_test.py

@@ -146,7 +146,8 @@ class TestMemmgr(unittest.TestCase):
                            self.__mgr._datasrc_config_handler)],
                          self.__mgr.mod_ccsession.add_remote_params)
 
-        # If data source isn't configured it's considered fatal.
+        # If data source isn't configured it's considered fatal (checking the
+        # same scenario with two possible exception types)
         self.__mgr.mod_ccsession.add_remote_exception = \
             isc.config.ModuleCCSessionError('faked exception')
         self.assertRaises(isc.server_common.bind10_server.BIND10ServerFatal,

src/lib/python/isc/memmgr/datasrc_info.py

@@ -146,12 +146,12 @@ class MappedSegmentInfo(SegmentInfo):
 
     def switch_versions(self):
         # Swith the versions as noted in the constructor.
-        self.__writer_ver ^= 1
+        self.__writer_ver = 1 - self.__writer_ver
 
         if self.__reader_ver is None:
             self.__reader_ver = 0
         else:
-            self.__reader_ver ^= 1
+            self.__reader_ver = 1 - self.__reader_ver
 
         # Versions should be different
         assert(self.__reader_ver != self.__writer_ver)