Support snapshot directly on VDIs

[julien-f]

• in VM / disks (should have the proper snapshot icon)
• vm.coffee/checkPermissionOnSrs

[olivierlambert]

Is it still relevant?

[julien-f]

Yes, VDI snapshots are not currently support in XO.

Whether we want to support it or not is a choice.

[olivierlambert]

We should have this indeed (we've seen bugs where you have sometimes a VDI snapshot attached, it's better to see it than just make it invisible).