SCALE: Add option to export/import VM disks

I've created a post on the forum with suggestions to improve the UX/UI for user focused on Virtualization:

https://www.truenas.com/community/threads/are-these-virtualization-features-on-the-roadmap.90063/

I'll split the suggestions on multiple tickets focused on specific features.

(3) Storage

Currently on SCALE there is no option to export/import VM disks to/from other hypervisors. This feature is already offered by other hypervisors and it's even used by software developpers and even Linux distos to distribute their base VM images.

As an example on XCP-ng, next to each disk attached to a VM there is a button to export that disk in VHD format: