virdbus: Unref the D-Bus connection when closing

As documented at
https://dbus.freedesktop.org/doc/api/html/group__DBusConnection.html#ga2522ac5075dfe0a1535471f6e045e1ee
the creator of a non-shared D-Bus connection has to release the last
reference after closing for freeing.
Signed-off-by: NMarc Hartmayer <mhartmay@linux.ibm.com>
Reviewed-by: NBjoern Walk <bwalk@linux.ibm.com>
Reviewed-by: NBoris Fiuczynski <fiuczy@linux.ibm.com>
Reviewed-by: NStefan Zimmermann <stzi@linux.ibm.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>

src/util/virdbus.c

@@ -144,6 +144,7 @@ void virDBusCloseSystemBus(void)
 {
     if (systembus && !sharedBus) {
         dbus_connection_close(systembus);
+        dbus_connection_unref(systembus);
         systembus = NULL;
     }
 }