[avalanche] Fix the getavalanchepeerinfo help

Summary:
This is missing the nodecount field.
Also return the node count before the list of nodes for convenience.

Ref T1635.

Test Plan:

bitcoin-cli help getpeerinfo

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Maniphest Tasks: T1635

Differential Revision: https://reviews.bitcoinabc.org/D9841