hw/cxl: Fix missing reserved data in CXL Device DVSEC

hw/cxl: Fix missing reserved data in CXL Device DVSEC

The r3.1 specification introduced a new 2 byte field, but
to maintain DWORD alignment, a additional 2 reserved bytes
were added. Forgot those in updating the structure definition
but did include them in the size define leading to a buffer
overrun.

Also use the define so that we don't duplicate the value.

Fixes: Coverity ID 1534095 buffer overrun
Fixes: 8700ee15de ("hw/cxl: Standardize all references on CXL r3.1 and minor updates")
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20240308143831.6256-1-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>