libvirtconnection: Do not start event loop on import

Starting a thread on import is very bad idea. This patch fix that by
adding start_event_loop() function and makes the event loop class
private and more robust.

Change-Id: I7da5c1ac00dd73d8788a28d606534384ea04c867
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22322
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Allon Mureinik <amureini@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>