virt: Fix import error when AUTOTEST_PATH is relative and work dir is changed.

Signed-off-by: Jiří Župka <jzupka@redhat.com>
run: Fix bug in commit efadf86

So, turns out that, when not running under autotest,
the AUTOTEST_PATH directory will be None, and that will
make os.path.abspath to fail. Let's apply the abspath
function only if that value is not None.

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