Support using pull upload mode when importing a OVA

[luisdavim]

The ovftool has a flag (--pullUploadMode) than when the argument for the OVA path is a URL allows skipping the local download to upload and the download is done directly by the ESXi host, it would be nice if this library would allow the same.

[dougm]

That would be nice, similar to govc library.import -pull. Hadn't seen this before, but looks like we'd need to use the pullFromUrls method