qtest: fix vhost-user-test compilation with old GLib Mising G_TIME_SPAN_SECOND definition breaks the RHEL6 compilation as GLib version before 2.26 does not have it. In such case just define it. Reported-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Nikolay Nikolaev <n.nikolaev@virtualopensystems.com> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>