get_container_interfaces() looked for ipconfigN keys, which exist only on
QEMU cloud-init guests. LXC keeps the address inside the netN string, so
the method never returned anything and containers showed no IP at all.
Parse netN with the existing net_parser, fall back to
/nodes/{node}/lxc/{vmid}/interfaces for DHCP guests (skipped when the
config already carries a static address or the container is stopped), and
validate addresses so loopback, link-local and the literal "dhcp" are
dropped. IPv6 from ip6/inet6 is reported as well.
