s390x: add test for (v)malloc

Let's test if basic allocation works and we get virtual addresses.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180213162321.20522-11-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>