Fix import/export of registry.xml.

[mbaechtold]

This fixes a problem in the test setup of an existing project: after running the tests of one test layer an error occurred during the tear down of it, before setting up another test layer.

For example:
Setting up one layer applies the kind of broken registry.xml and before setting up the second layer the first layer tears down the wrong way (Don't know whats wrong) and introduces the problem in setting up the second layer.

ConnectionStateError: Shouldn't load state for 0x3a5192e7a775ec12 when the connection is closed

Running zope.testrunner.layer.UnitTests tests:
  Tear down plone.app.testing.layers.PloneFixture in 0.067 seconds.
  Tear down plone.testing.z2.Startup 
Traceback (most recent call last):
  File "bin/test", line 447, in <module>
    sys.exit(collective.xmltestreport.runner.run_internal(['-s', 'izug.organisation', '--exit-with-status', '--auto-color', '--auto-progress', '--xml', '--test-path', '/Users/mbaechtold/Developer/Projects/izug.organisation.simplelayout']))
  File "/Users/mbaechtold/.buildout/eggs/collective.xmltestreport-1.3.3-py2.7.egg/collective/xmltestreport/runner.py", line 73, in run_internal
    runner.run()
  File "/Users/mbaechtold/.buildout/eggs/zope.testrunner-4.4.4-py2.7.egg/zope/testrunner/runner.py", line 160, in run
    self.run_tests()
  File "/Users/mbaechtold/.buildout/eggs/zope.testrunner-4.4.4-py2.7.egg/zope/testrunner/runner.py", line 242, in run_tests
    self.skipped, self.import_errors)
  File "/Users/mbaechtold/.buildout/eggs/zope.testrunner-4.4.4-py2.7.egg/zope/testrunner/runner.py", line 394, in run_layer
    tear_down_unneeded(options, needed, setup_layers)
  File "/Users/mbaechtold/.buildout/eggs/zope.testrunner-4.4.4-py2.7.egg/zope/testrunner/runner.py", line 685, in tear_down_unneeded
    l.tearDown()
  File "/Users/mbaechtold/.buildout/eggs/plone.testing-4.1.1-py2.7.egg/plone/testing/z2.py", line 354, in tearDown
    self.tearDownBasicProducts()
  File "/Users/mbaechtold/.buildout/eggs/plone.testing-4.1.1-py2.7.egg/plone/testing/z2.py", line 676, in tearDownBasicProducts
    with zopeApp() as app:
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/contextlib.py", line 17, in __enter__
    return self.gen.next()
  File "/Users/mbaechtold/.buildout/eggs/plone.testing-4.1.1-py2.7.egg/plone/testing/z2.py", line 276, in zopeApp
    app = addRequestContainer(Zope2.app(connection), environ=environ)
  File "/Users/mbaechtold/.buildout/eggs/Zope2-2.13.24-py2.7.egg/Zope2/__init__.py", line 52, in app
    return bobo_application(*args, **kw)
  File "/Users/mbaechtold/.buildout/eggs/Zope2-2.13.24-py2.7.egg/App/ZApplication.py", line 75, in __call__
    return connection.root()[aname]
  File "/Users/mbaechtold/.buildout/eggs/ZODB3-3.10.5-py2.7-macosx-10.11-x86_64.egg/ZODB/Connection.py", line 366, in root
    return RootConvenience(self.get(z64))
  File "/Users/mbaechtold/.buildout/eggs/ZODB3-3.10.5-py2.7-macosx-10.11-x86_64.egg/ZODB/Connection.py", line 248, in get
    p, serial = self._storage.load(oid, '')
AttributeError: 'NoneType' object has no attribute 'load'