virttest.aexpect: Change file/log directory structure

Instead of having a flat structure of files

BASE_DIR/[filename]-[aexpect_instance]

Make the aexpect instance the parent dir of the files

BASE_DIR/[aexpect_instance]/filename

That makes it easy to see the number of open aexpect
controlled process at any given point in time, while
keeping information available.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>