Regression: Mender 1.7.0 misidentifies indexes of LVM volumes

Due to the fix to enable partuuid, there is a regression which causes Mender to misidentify the indexes of LVM volumes: The symbolic link contains a stable index (e.g. /dev/mender/rootfs1), but Mender resolves this link to the underlying file, which does not have a stable index (e.g. /dev/dm-0).

The discussion in the pull request gives most of the context, and this comment gives a proposed solution.