Code review comment for lp:~javier.collado/utah/bug1087620

I've tested again this branch and this time I haven't found any problem like
the last time.

ISO validation works for all images I've tested (server, desktop, i386, amd64)
and the `pass.run` runlist works fine for desktop i386, so this doesn't seem ot
break something that depends on `dump` or `extractall`.

The reason I'm not happy with the changes is because the extract `getrealfile`
method doesn't reproduce symbolic links as the old version used to do. Anyway,
I find the implementation is clearer now and it can be refactored later to be
more efficient with regard to hard/symbolic links.