Makes some changes in order to properly generate VMware OEM images

* Upgrades virtual hardware version to 7 which is the minimal version required in order to be able to use vmxnet3 network drivers, serial attached SCSI virtual devices and IDE devices. Version 7 was introduced with vSphere 4.0 

* Defines oem-vmware package so it is installed and built along with its dependencies when generating VMware images.

* Modifies `vm_image_util` script to fix an issue where oem-vmware package dependencies wouldn't get installed at all. This change, especially, might require some discussion as I'm sure I'm ignoring some assumptions made by CoreOS team. I'm hoping this change won't break the generation of the rest OEM images, but I didn't really test them.

This PR is part of a larger change-set intended to create a good starting point for VMware OEM images. The change-set is composed of the following PRs: https://github.com/coreos/scripts/pull/336, https://github.com/coreos/coreos-overlay/pull/898, https://github.com/coreos/init/pull/137, https://github.com/coreos/portage-stable/pull/139
Sorry, but modifications outside of `/usr/share/oem` should only be done during the `build_image` stage, `image_to_vm` should avoid modifying that common base.