s390x/mmu: Factor out storage key handling

Factor it out, add a comment how it all works, and also use it in the
REAL MMU.

Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20190816084708.602-7-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>