s390x: add basic migration test

Add a basic migration test for s390x. This tests the guarded-storage
registers and vector registers are preserved on migration.

Check for the respective facilities. Since it is possible neither
guarded-storage nor vector are there, add an additional report_pass()
such that least one PASS is reported.

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