mockmodel.py: unsubscribing from 'exit' channel on cleanup

The function virtviewertmpfile_cleanup executes the cleanup
of the temporary file that the virt viewer mockmodel uses. For
each instance of the Mockmodel class, a new file is created and
needs cleanup.

When running the unit tests in WoK, this function gives an error
saying that the file doesn't exist. Further investigation showed
that the same cleanup function was being called multiple times,
causing this error.

This happens because the cherrypy 'exit' channel is process
wide and its subscribers are kept in subsequent calls of
cherrypy.engine.exit(). This scenario is common when running
the current WoK tests, which creates multiple server instances
in the same process.

The sitation before this patch can be exemplified as follows:

- Server A is instanced. The function 'cleanA' is subscribed
in the Cherrypy 'exit' channel;

- Server A exits. 'cleanA' is executed but it is not unsubscribed
from the 'exit' channel;

- A new server B is instanced. A function 'cleanB' is subscribed
in the 'exit' channel;

- Server B exits. A message is sent to all subscribers of the 'exit'
channel. 'cleanA' is called again and throws an error because there is
nothing else to clean up from the previous server A.

Making the cleanup function unsubscribe itself from the 'exit' channel
solves the issue.

This fixes WoK bug kimchi-project/wok#183

Signed-off-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>

mockmodel.py

@@ -157,6 +157,7 @@ def _create_virt_viewer_tmp_file(self):
 
     def virtviewertmpfile_cleanup(self):
         os.unlink(self.virtviewerfile_tmp.name)
+        cherrypy.engine.unsubscribe('exit', self.virtviewertmpfile_cleanup)
 
     def reset(self):
         MockModel._mock_vms = defaultdict(list)