Fix maxSize test when setting device target size.
Fix maxSize test when setting device target size.

maxSize can be 0, which means there is no max size.

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