backup: factor out & clean up gathering device info into helper

backup: factor out & clean up gathering device info into helper

Squash the two original patches [0][1] from Fiona, which got send
separate to be easier to review, into the big patch that adds the
Proxmox backup integration.

[0]: https://lists.proxmox.com/pipermail/pve-devel/2024-January/061479.html
[1]: https://lists.proxmox.com/pipermail/pve-devel/2024-January/061478.html

Originally-by: Fiona Ebner <f.ebner@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>