make check-venv may leave stale and incomplete tests/venv directory directory

As reported by "Philippe Mathieu-Daudé" <email address hidden>, a "make check-venv" can be run and fail to properly create a suitable virtual environment, leaving the tests/venv directory which is the target for "make check-venv" itself.

This means that on a subsequent run: 

> $ make check-venv
>   GIT     ui/keycodemapdb tests/fp/berkeley-testfloat-3
> tests/fp/berkeley-softfloat-3 dtc capstone slirp
> make: Nothing to be done for 'check-venv'.

And the venv will still be incomplete.  The causes of such failures to create a suitable virtual environment are too many (in the reported case it was because of missing *required* Python packages).  Some more evolved virtual environments + Python packaging systems exist that could probably be used here (Pipenv) but would add further core requirements.

The current mitigation is to run "make check-clean" when the venv appears to be incomplete.

The goal of this bug is to attempt to make the venv setup atomic and more reliable.