Is your feature request related to a problem? Please describe.
Set a default value for User-Agent in http.Client when using the library (govc already allows this) to identify requests coming from govmomi SDK (currently it only shows a generic Go client in stats). Useful for debugging, statistics, etc.

Describe the solution you'd like
Set a default value for User-Agent in http.Client

Describe alternatives you've considered
Keep as is

Additional context
Should not be considered a breaking change IMHO?