run: Kill vms and clean tmp files on Ctrl+C handler

Make the cleanup_env method more generic, and add it
to the KeyboardInterrupt handler of virt-test. This
way, users that do kill their virt-test execution
will have all vms and temporary files properly
cleaned.

This way, no temporary files will be around even
on Ctrl + C termination.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>