Fix the name of the variable specifying requested libblockdev plugins (#1256273)
Fix the name of the variable specifying requested libblockdev plugins (#1256273)

This got mangled as part of the merge.

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