builder/virtualbox,vmware: add logging to http server [GH-906]

Fixes #906.

Log format is:
````
Received HTTP request: [192.168.6.228:34412] GET /preseed.cfg 200
````
I don't see an easy way to include the size of the data returned.

If #990 doesn't now merge cleanly, let me know, and I will fix and resubmit.