Do not try to get format free space for non-existing formats
Do not try to get format free space for non-existing formats

Resolves: rhbz#1676480

rvykydal/blivet,vojtechtrefny/blivet,jkonecny12/blivet,rvykydal/blivet,vojtechtrefny/blivet,jkonecny12/blivet