hw/cxl: Add stub write function for RO MemoryRegionOps entries.

hw/cxl: Add stub write function for RO MemoryRegionOps entries.

There is no checking on the availability of a write callback.
Hence QEMU crashes if a write does occur to one of these regions.

Discovered whilst chasing a Linux kernel bug that incorrectly
wrote into one of these regions.

Fixes: 6364adacdf ("hw/cxl/device: Implement the CAP array (8.2.8.1-2)")
Reported-by: Bobo WL <lmw.bobo@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20220817145759.32603-2-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>