s390x: add support for migration tests

Now that we have SCLP console read support, run our tests with migration_cmd, so
we can get migration support on s390x.

Signed-off-by: Nico Boehr <nrb@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>