virt: more aexpect unittests

Signed-off-by: Chris Evich <cevich@redhat.com>
virt: aexpect needn't only reference fd integers

Signed-off-by: Chris Evich <cevich@redhat.com>
virt: aexpect should validate parameters

Signed-off-by: Chris Evich <cevich@redhat.com>