Fix symlink confusion for downloaded VM images

Previous code would download e.g. slax.iso to $prefix/slax.iso/slax.iso, then try to symlink it to $prefix/slax.iso (which wouldn't work because the directory already exists with the same name). Unfortunately, the CGI would look for $prefix/slax.iso, which is a directory.

The patch fixes that, by downloading the file into prefix/downloads/slax.iso/slax.iso then symlinking it into the right place (prefix/slax.iso).