client.shared: Remove pexpect module and update code to use aexpect module.

This modifies big parts of Autotest to remove previous dependence on pexpect
and then making use of virttest's aexpect module.

So basically we are:

* Removing pexpect.py and pxssh.py modules.
* Importing modules aexpect.py, data_dir.py, rss_client.py and remote.py
  from virttest.
* Importing selected functions from virttest's utils_misc.py into utils.py.
* Updating/removing references from pexpect to aexpect.
* Make ssh_key module to use aexpect and remote.
  - Also: function setup_ssh_key(): default port is 22.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #810 from ruda/use_aexpect

client.shared: Remove pexpect module and update code to use aexpect module