virttest.aexpect: Introduce server side logging

Make the aexpect server able to log into a file. As it
is, we don't really have an easy way to tell what's going
on in there.

On later patches, we'll implement better ways to manage
files generated by aexpect, as well as debugging mechanisms.

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