hw/mem/cxl_type3: Add host backend and address space handling for DC regions

hw/mem/cxl_type3: Add host backend and address space handling for DC regions

Add (file/memory backed) host backend for DCD. All the dynamic capacity
regions will share a single, large enough host backend. Set up address
space for DC regions to support read/write operations to dynamic capacity
for DCD.

With the change, the following support is added:
1. Add a new property to type3 device "volatile-dc-memdev" to point to host
   memory backend for dynamic capacity. Currently, all DC regions share one
   host backend;
2. Add namespace for dynamic capacity for read/write support;
3. Create cdat entries for each dynamic capacity region.

Reviewed-by: Gregory Price <gregory.price@memverge.com>
Signed-off-by: Fan Ni <fan.ni@samsung.com>
Message-Id: <20240523174651.1089554-9-nifan.cxl@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>