According to the architecture, SET PREFIX must try to access the new
prefix area and recognize an addressing exception if the area is not
accessible.
For qemu this check prevents a crash in cpu_map_lowcore after an
inaccessible prefix area has been set.

Signed-off-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20220630094340.3646279-1-scgl@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>