Revert "vdisk list: only collect images from storages with an appropriate content...

Revert "vdisk list: only collect images from storages with an appropriate content type"

This reverts commit a44c18925d223a971296801a0985db34707ada4d and adds a reminder
comment.

The mentioned commit is actually a backwards-incompatible change that leads to
slightly different behavior when migrating a VM with volumes on a misconfigured
storage. For example, unreferenced volumes on a misconfigured storage won't be
picked up, even though they were before. And for referenced volumes on a
misconfigured storage, the disk size would not be updated on migration anymore.

We should wait until the next major release for this change and then also
re-evaluate the migration behavior with misconfigured disks.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>