if run in a virtualenv and cannot find pyuno at startup, show reminder a...

...bout uno.py/unohelper.py presence in virtualenv site-packages dir

* by default virtualenv isolates from system site-packages
* uno.py / unohelper.py from package python-uno are in /usr/share/pyshared and thus not available in virtualenv

This purpose of this patch is to help user with a possible fix when unoconv can't start in a virtualenv