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A proposal for #26 : allow customer to specify (any ?) (kernel-only ?) disks for vm creation.

A "system" or "data" disk type does not really exist (deprecated) - the kernel/cmdline attributes are the real filter here, but not implemented in this proposal.

We also lack the datacenter location for disks, that should help select the right image.
Choices should only contains names (strings) so it's more user friendly to manipulate directly (than to remember ids), and it will be translated later during a usable_id call in modules.

We tried to apply this same logic to all choices type parameters (like datacenters)