virttest.aexpect: Do not remove the aexpect dir wholesale

On double .close(), we'll get to a point where there's an
aexpect instance, but its parent directory was already
nuked. This will cause the .close() method to try to open
a log file in a path where the parent directory doesn't
exist anymore.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>