Add demo-mode to KVM.

To use it:

1: Create an admin disk by running kvm-admin with --create-image <file>
2: Setup the admin node as normal.
3: Once the admin node has converged:
   1: rm /etc/udev/rules.d/*persist*
   2: halt
4: After the system has halted, kill the kvm-admin script.  This
   will cause the disk the admin node was using to be converted into
   a compressed qcow2 image stored in <file>
5: When you are ready to run the demo, run kvm-admin --demo <file>