Fix WindowsMixin._GetNumCpus when more than one Win32_processor exists.
Fix WindowsMixin._GetNumCpus when more than one Win32_processor exists.

AdamIsrael/PerfKitBenchmarker,AdamIsrael/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,meteorfox/PerfKitBenchmarker,meteorfox/PerfKitBenchmarker