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.
Couldn't this just replace the use of qemu-img in _write_vmdk_scsi_disk?