*) always replace old cloudinit images
*) apply pending cloudinit changes when generating a new
image
For cloudinit we now always use vdisk_free before
vdisk_alloc in order to always replace old images, this
allows us to hotplug a new drive by setting it to
`none,media=cdrom` first (to eject the disk), then setting
it back to 'storage:cloudinit' to have a new image generated
after applying the currently pending changes.