[snapshot][info] Snapshot size not provided

[tobe2098]

Describe the bug
Snapshot size is not populated. The field is unused by the daemon.

To Reproduce
How, and what happened?

1. multipass launch -n a
2. multipass stop a
3. multipass snapshot a
4. multipass info --snapshots --format json

Expected behavior
The field should either not be present or be populated.

Additional info

• OS: Ubuntu 24.04
• CPU architecture or model: amd64
• multipass version: 1.17.0-dev