This was added in bd9814f0db to support downgrading
docker 1.7 to 1.6.
The related migration code was removed in 0023abbad3
(Docker 18.05), which was also the last consumer of VolumeDataPathName outside
of the package, so that const can be un-exported.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>