hw/acpi: Do not create memory hotplug method when handler is not defined

hw/acpi: Do not create memory hotplug method when handler is not defined

With Hardware-reduced ACPI, the GED device will manage ACPI
hotplug entirely. As a consequence, make the memory specific
events AML generation optional. The code will only be added
when the method name is not NULL.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20190918130633.4872-3-shameerali.kolothum.thodi@huawei.com>
Acked-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>