s390x: detect installed memory

Unfortunately, there is no easy way to simply read out the amount
of installed memory. We have to probe (via TEST PROTECTION) for installed
memory in a given range.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180213162321.20522-7-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>