s390x/tcg: low-address protection support

This is a neat way to implement low address protection, whereby
only the first 512 bytes of the first two pages (each 4096 bytes) of
every address space are protected.

Store a tec of 0 for the access exception, this is what is defined by
Enhanced Suppression on Protection in case of a low address protection
(Bit 61 set to 0, rest undefined).

We have to make sure to to pass the access address, not the masked page
address into mmu_translate*().

Drop the check from testblock. So we can properly test this via
kvm-unit-tests.

This will check every access going through one of the MMUs.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20171016202358.3633-3-david@redhat.com>
[CH: restored error message for access register mode]
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
