A VM overlay is a binary delta that contains user-customized part of
the virtual machine. In practice, it is a single zip-formatted file.
For example, if your VM is running a face recognition server on Ubuntu
12.04 LTS, all the bits related to the installation of the face
recognition server will be encapsulated into a VM overlay. This VM
overlay contains all the state of your application and it is delivered
to the cloudlet to provision the backend server using VM synthesis