Acceptance test x86_cpu_model_versions: shutdown VMs

Acceptance test x86_cpu_model_versions: shutdown VMs

This shuts down the VMs that won't be used any longer during the
remainder of the test.

It's debatable if the very last one should also be shutdown manually,
and my opinion is that it shouldn't because that's taken care by the
immediately following tearDown().

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20190828193628.7687-2-crosa@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>