Guard dbus.start() with dbus.is_running()
    
    Some platforms like Debian protect the dbus.service with
    RefuseManualStart=True. "systemctl start dbus" fails with operation
    refused (it is configured to refuse manual start/stop). On Fedora
    "systemctl start dbus" is a no-op when dbus is already running.
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>