govc: fix host.info CPU usage
govc: fix host.info CPU usage

From the doc HostListSummaryQuickStats:

> overallCpuUsage - Aggregated CPU usage across all cores on the host in MHz.

Use NumCpuCores instead of NumCpuThreads.

Fixes #1878

vmware/govmomi,bruceadowns/govmomi,bruceadowns/govmomi,vmware/govmomi,vmware/govmomi,bruceadowns/govmomi