tests.acceptance: adds simple migration test

This change adds the simplest possible migration test. Beyond the test
purpose itself it's also useful to exercise the multi virtual machines
capabilities from base avocado qemu test class.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Signed-off-by: Caio Carrara <ccarrara@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20190212193855.13223-3-ccarrara@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>