Persistent Libvirt Connections

Implements connection pooling. Only one connection per vm-host per webserver-process is created and managed by a new class "wvmConnectionManager".
Outside of the vrtManager.connection module the old behaviour is preserved.
This is tested with apache2 using mod_wsgi and the Django development server. Nginx was not tested.
The branch https://github.com/myvision-de/webvirtmgr/tree/connection-pooling-debug may be useful when testing this PR with another server.

Commit c8f0293 is optional. It prohibits some further log spamming in auth.log. It may be more practical to evaluate the error code from libvirt to determine if authentication fails or the server is just down.