s390x: uv-host: Fence a destroy cpu test on z15

Firmware will not give us the expected return code on z15 so let's
fence it for the z15 machine generation.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Acked-by: Thomas Huth <thuth@redhat.com>