tests/qtest/migration-test: Disable the analyze-migration.py test on s390x

The analyze-migration.py script fails on s390x hosts:

 Traceback (most recent call last):
   File "scripts/analyze-migration.py", line 662, in <module>
     dump.read(dump_memory = args.memory)
   File "scripts/analyze-migration.py", line 596, in read
     classdesc = self.section_classes[section_key]
 KeyError: ('s390-storage_attributes', 0)

It obviously never has been adapted to s390x yet, so until this
has been done, disable this test on s390x.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20231018091239.164452-1-thuth@redhat.com>