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>
Tested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>