add vmware_ova to image formats

This adds code to vm_image_util.sh to create ova bundles, suitable to be deployed in VMware ESXi, Workstation and Fusion. This PR depends on https://github.com/coreos/portage-stable/pull/157. It addresses issue https://github.com/coreos/bugs/issues/176.
No, the tool vmdk-convert needs a vmdk as input.