[avalanche] Fix type error in abc_p2p_avalanche_peer_discovery

Summary: The inv field of a getdata message is actually a list. The method was unused so the test didn't fail, but it will be used in D13029.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_peer_discovery

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13042