virttest.aexpect: Fix old class Spawn definition.

The base class Spawn for ShellSession was defined in old way(class Spawn).
Then we can not use super() for inherited classes:
TypeError: super() argument 1 must be type, not classobj

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
README.rst: add basic troubleshooting instructions

As suggested by Juan Quintela.

CC: Juan Quintela <quintela@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>