DockerPropertiesV8 now accepts custom_adapters (already available to
v1-6 top-level appliances and to Qemu v8 properties), so port-named
Docker appliances (XRd, SR Linux) can move to the v8 format without
losing their interface naming.
Appliance.type resolves the node type from the v8 settings template_type
(default set first) instead of misclassifying every v8 appliance as
qemu, and _get_default_symbol applies the docker guest symbol to v8
Docker guest appliances.
