cxl/test: Enhance event testing

An issue was found in the processing of event logs when the output
buffer length was not reset.[1]

This bug was not caught with cxl-test for 2 reasons.  First, the test
harness mbox_send command [mock_get_event()] does not set the output
size based on the amount of data returned like the hardware command
does.  Second, the simplistic event log testing always returned the same
number of elements per-get command.

Enhance the simulation of the event log mailbox to better match the bug
found with real hardware to cover potential regressions.

NOTE: These changes will cause cxl-events.sh in ndctl to fail without
the fix from Kwangjin.  However, no changes to the user space test was
required.  Therefore ndctl itself will be compatible with old or new
kernels once both patches land in the new kernel.

[1] Link: https://lore.kernel.org/all/20240401091057.1044-1-kwangjin.ko@sk.com/

Cc: Kwangjin Ko <kwangjin.ko@sk.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20240401-enhance-event-test-v1-1-6669a524ed38@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

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