(hopefully) fix the memory issues in the VMs

(hopefully) fix the memory issues in the VMs

- Create and populate an host extension, used by the VM plugin to
  check the total amount of memory of that host and whether this host
  allows overcommiting VM above this value.
- This is a great step toward the pluginization of the VMs
- Unfortunately, there is no way to change these values for an host
  yet :-|