s390x: add migration test for storage keys

Upon migration, we expect storage keys set by the guest to be preserved, so add
a test for it.

We keep 128 pages and set predictable storage keys. Then, we migrate and check
that they can be read back and match the value originally set.

Signed-off-by: Nico Boehr <nrb@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Message-Id: <20220614110521.123205-2-nrb@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>