Export buildah validate volume functions so it can share code with libpod
Export buildah validate volume functions so it can share code with libpod

Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>

Closes: #1560
Approved by: giuseppe

nalind/buildah,nalind/buildah-1,rhatdan/buildah,projectatomic/buildah,TomSweeneyRedHat/buildah,TomSweeneyRedHat/buildah,projectatomic/buildah,nalind/buildah,rhatdan/buildah,rhatdan/buildah,projectatomic/buildah,nalind/buildah-1,nalind/buildah-1,TomSweeneyRedHat/buildah