Support CSV format response in QueryPerf API

[jsleblanc]

Is your feature request related to a problem? Please describe.
QueryPerf API PerfQuerySpec argument allows clients to specify the return format for performance counter data ("normal", "csv"); using CSV format is recommended by vmware for performance reasons, but govcsim does not support the format property at this time. Clients that request performance counter data in CSV format will not receive data in the format they expect.

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

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.Format.html

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.EntityMetricCSV.html

Describe the solution you'd like
When format is specified as "csv" on the PerfQuerySpec argument to QueryPerf, return performance counter data in CSV format, otherwise return data in the "normal" format - the behaviour of govcsim today.

Describe alternatives you've considered
Behaviour is defined by the vmware API; this is just a small gap in govcsim implementation at this time.