Use key_name instead of link_id
Some entities do not have a link_id, but all entities are guaranteed
to have a key_name (or an id).
<OGRVRTDataSource>
<OGRVRTLayer name="test_vrt">
<SrcDataSource relativeToVRT="1">test_vrt.csv</SrcDataSource>
<GeometryType>wkbPoint</GeometryType>
<GeometryField encoding="PointFromColumns" x="POINT_X" y="POINT_Y" z="NUM"/>
</OGRVRTLayer>
</OGRVRTDataSource>