Option to add separate data disk, also for the disk-booting instances?

### Description

Currently we have one "alpine-iso" set of images, that boot from ISO and persists to a data disk.

Then we have the "alpine-image" (and all the rest), that boot from HDD and persists to a diff disk*

\* at best, assuming QEMU QCOW used - otherwise base disk and diff disk is merged to "raw"

It could be nice with an option to keep `/var/lib` on a separate disk, different from the OS disk?

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Otherwise it leads to some consequences like the disk being "already used" for `system df`

You _could_ expect that since you haven't installed any images or containers, it should be zero...

But instead you are seeing the `df` of the allocated (empty) disk, and filled with the base OS.

It also might make it easier to backup or recover, if something should happen to the OS.