s390x/tcg: specification exception for unknown diag

While the PoP is silent on the issue, z/VM documentation states
that unknown diagnose codes trigger a specification exception.
We already do that when running with kvm, so change tcg to do so
as well.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>