agent.network_get_interfaces.get() built the URL path segment literally
("network_get_interfaces"), but the real Proxmox REST endpoint uses
hyphens ("network-get-interfaces") and must be reached via agent(...) as
a callable resource — the underscored attribute path 404ed silently on
every poll, so wait_for_ip always ran out the full timeout even though
the guest agent was reporting the IP to Proxmox correctly the whole time.
Also stopped assuming interfaces[0] is the real NIC — the guest agent
commonly reports "lo" first, matching the working pattern already used
in vm_mixin.py (skip "lo", require ip-address-type == "ipv4").