Datastore upload and download is inefficient

[jnewbigin]

http://www.virtuallyghetto.com/2014/06/how-to-efficiently-transfer-files-to-datastore-in-vcenter-using-the-vsphere-api.html summarises the problem.
I have implemented the so called efficient mode in knife-vsphere but it seems like it belongs here in RbVmomi::VIM::Datastore
If you are interested I can re-factor my change and open a pr here.

[jrgarcia]

I'll never turn down a PR that improves efficiency! Feel free to open! 👍