Resolve VIRTUAL_ENV if it is a symlink

All paths checked against VIRTUAL_ENV are being resolved, therefore
VIRTUAL_ENV should be resolved, too.

It does not solve my initial problem, where I had two symlinks though:
~/.virtualenvs/project => /srv/project/virtualenv
and /srv/project => /home/www-data/project

The real virtualenv is /home/www-data/project/virtualenv, but os.path.readlink only resolves the first level.