[BUG] QueryPerf PerfQuerySpec.MaxSample is optional, but treated as provided by clients when it isn't

[jsleblanc]

Describe the bug
When querying performance counters using the QueryPerf method, the PerfQuerySpec argument defines an optional property called "maxSample" that will limit the number of samples returned. The documentation indicates that this property is optional and does not need to be specified by clients on their request. The issue is govcsim defines this property as a non-nullable integer, so clients that do not specify a value will result in maxSample defaulting to 0 and no data will be returned to the client.

https://vdc-repo.vmware.com/vmwb-repository/dcr-public/c476b64b-c93c-4b21-9d76-be14da0148f9/04ca12ad-59b9-4e1c-8232-fd3d4276e52c/SDK/vsphere-ws/docs/ReferenceGuide/vim.PerformanceManager.QuerySpec.html

To Reproduce
Steps to reproduce the behavior:

1. Make a call to QueryPerf for any entity/metric, specify entity, metricId, intervalId, startTime, endTime, but do not provide a value for "maxSample" on the PerfQuerySpec argument.

Expected behavior
QueryPerf should return simulated data, but sampleInfo on every PerfEntityMetric in the response is null.

Affected version
HEAD of main

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