FillDefault updates undefined fields in y with defaults from d (or built-in default), and overwrites with values from o.

FirstUsernetIndex gets the index of first usernet network under l.Network[].

Load loads the yaml and fulfills unspecified fields with the default values.

Save saves the yaml.

ValidateParamIsUsed checks if the keys in the `param` field are used in any script, probe, copyToHost, or portForward.

Default9pSecurityModel is "none" for supporting symlinks https://gitlab.com/qemu-project/qemu/-/issues/173.