hw/cxl: Check enough data in cmd_firmware_update_transfer()

hw/cxl: Check enough data in cmd_firmware_update_transfer()

Buggy guest can write a message that advertises more data that
is provided. As QEMU internally duplicates the reported message
size, this may result in an out of bounds access.
Add sanity checks on the size to avoid this.

Reported-by: Esifiel <esifiel@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20241101133917.27634-5-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>