Log invalid memory accesses with as GUEST_ERROR.

This is particularly useful since commit 5d971f9e which reverted
("memory: accept mismatching sizes in memory_region_access_valid").

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20201005152725.2143444-1-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>