[avalanche] Move the vote items to the update structure

Summary:
The VoteItemUpdate constructor moves the item, but this item is const.
Fix the code to properly move the item where applicable.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12975