tools/testing/cxl: Mock support for Get Poison List

Make mock memdevs support the Get Poison List mailbox command.
Return a fake poison error record when the get poison list command
is issued.

This supports testing the kernel tracing and cxl list capabilities
for media errors.

Signed-off-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/14d661ce3e3a32b7d8e76b8ecc5eb88343b3d09c.1681838292.git.alison.schofield@intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>