feat(vm_image_util): Add new Rackspace target with VHD disk images.
feat(vm_image_util): Support other disk formats with Xen, such as vhd.
fix(build_image): Add auto-enter-chroot call

This makes build_image and image_to_vm behave like build_packages, where
if you call the script outside of the SDK's chroot it will automatically
reexecute itself inside of the chroot.
Merge pull request #203 from marineam/vhd

VHD disk images