s390x/tcg: add MMU for real addresses

This makes it easy to access real addresses (prefix) and in addition
checks for valid memory addresses, which is missing when using e.g.
stl_phys().

We can later reuse it to implement low address protection checks (then
we might even decide to introduce yet another MMU for absolute
addresses, just for handling storage keys and low address protection).

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170926183318.12995-3-david@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
