dbus: register object manager object before requesting dbus name

Working on NetworkManager 1.12.4 and sometimes (rarely), when creating a NM client object before NetworkManager service start, this object will never be running. In that case, we can see the following log: "[GLIB-GLib-GIO WARN] Error calling GetManagedObjects() when name owner :1.5 for name org.freedesktop.NetworkManager came back: GDBus.Error:org.freedesktop.DBus.Error.UnknownMethod: No such interface 'org.freedesktop.DBus.ObjectManager' on object at path /org/freedesktop".

Object Manager object shall be registered before requesting dbus name to be sure that 'org.freedesktop.Dbus.ObjectManager' interface is present when name owner change is received by libnm.