In our wrapper of g_dbus_connection_call_sync() in
virfirewalltest a string is duplicated and added onto a
virStringList. This leads to a memory leak because
virStringListAdd() duplicates the string itself.
Signed-off-by: Michal Privoznik <mprivozn@redhat.com> Reviewed-by: Pavel Hrdina <phrdina@redhat.com>