tools/testing/cxl: Make cxl_memdev_state available to other command emulation

Move @mds out of the event specific 'struct mock_event_store' and into
the base 'struct cxl_mockmem_data' directly. This is in preparation for
enabling cxl_test to exercise the notifier flow for 'sanitize' operation
completion.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>