hw/cxl/cxl-mailbox-utils: remove unneeded mailbox output payload space zeroing

hw/cxl/cxl-mailbox-utils: remove unneeded mailbox output payload space zeroing

The whole mailbox output payload space is already zeroed after copying
out the input payload, which happens before processing the specific mailbox
command:
https://elixir.bootlin.com/qemu/v8.2.1/source/hw/cxl/cxl-device-utils.c#L204

Signed-off-by: Fan Ni <fan.ni@samsung.com>
Link: https://lore.kernel.org/r/20240221221824.1092966-1-nifan.cxl@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20240705113956.941732-3-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>