engine: cannot set payload and cloud-init on same devices.

On initial run we automatic set cloud-init/sysprep as cdrom/floppy
device - this cause conflict when using the same media.

Change-Id: Ic5b701f058a2d87b2836d337f822d2f012d10bbe
Bug-Url: https://bugzilla.redhat.com/1138314
Signed-off-by: Shahar Havivi <shaharh@redhat.com>