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>