Use a safer method to get a dm partition's disk name.
Use a safer method to get a dm partition's disk name.

Resolves: rhbz#1190886
Related: rhbz#1181336

AdamWill/blivet,AdamWill/blivet,rvykydal/blivet,jkonecny12/blivet,vojtechtrefny/blivet,jkonecny12/blivet,rhinstaller/blivet,rhinstaller/blivet,vojtechtrefny/blivet,dwlehman/blivet,dwlehman/blivet,vpodzime/blivet,rvykydal/blivet,vpodzime/blivet