feat(vm_image_util): Add /usr/.noupdate flag file in PXE images.

This can be used by update_engine as a quick test to determine if it is
running on a system that it can handle. This avoids needing something
like the 'coreos.diskless' kernel command line flag.
fix(vm_image_util): Default to larger vm (8GB) disk layout for GCE

GCE was using the base 3GB layout which isn't very useful for actual
machines.
Merge pull request #212 from marineam/gce

fix(vm_image_util): Default to larger vm (8GB) disk layout for GCE
Merge pull request #209 from marineam/pxe

feat(vm_image_util): Add /usr/.noupdate flag file in PXE images.
feat(iso): add iso image format to image_to_vm
refactor to use _write_cpio_common