Detect current virtualenv in a more compatible way

This broke down horribly during a bit of reorganization of how the tests
are run, and under which versions of virtualenv they run.

See https://stackoverflow.com/a/1883251 for details on why this needs to
change.

python/nav/config.py

@@ -42,7 +42,7 @@
     '/etc/nav',
     os.path.join(buildconf.datadir, 'conf'),
 ]
-_venv = os.getenv('VIRTUAL_ENV')
+_venv = sys.prefix if sys.prefix != sys.base_prefix else None
 if _venv:
     CONFIG_LOCATIONS = [
         os.path.join(_venv, 'etc'),