Stop mounting the installation disk

[ahahn94]

The installation image is always mounted at boot, leading to an annoying notification after startup.
Manually unmounting the installation disk from within the VM doesn't fix the problem, as the image will be remounted at the next system boot.
Maybe make mounting the installation disk an option which is on by default but turns off after installation or can be turned on/off by the user.

[popey]

Had a think and I've split the launch and install scripts out, so that it doesn't mount the installation media if the qcow is over a certain size, guessing the install is done.

(this should all be made nicer once there's a pretty graphical frontend, which currently is only in my imagination :D )