Disable 'cdrom0' device while ISO installation

In continue of the #3 discussions:
I reverted  ec95917.
Also, I suggest to disable 'cdrom0' and just enable it back during the cleanup step.

Disabling 'cdrom0', we are sure that this device will not be detected by guest OS, so that any conflicts are not possible. This is simpler than position switching.

I've tested this changeset on my patched OmniOS and FreeBSD templates from here: https://github.com/legal90/bento 
All is working fine.