hw/cxl: Create helper function to create DC Event Records from extents

hw/cxl: Create helper function to create DC Event Records from extents

Prepatory patch for following FMAPI Add/Release Patches. Refactors part
of qmp_cxl_process_dynamic_capacity_prescriptive() into a helper
function to create DC Event Records and insert in the event log.

Moves definition for CXL_NUM_EXTENTS_SUPPORTED to cxl.h so it can be
accessed by cxl-mailbox-utils.c and cxl-events.c, where the helper
function is defined.

Reviewed-by: Fan Ni <fan.ni@samsung.com>
Signed-off-by: Anisa Su <anisa.su@samsung.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20250714174509.1984430-10-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>