Introduce `VirtualMachineType` to classify virtual machines and apply
default platform, vCPU, and memory values when creating a VM.
This adds the new model and its relationship to `VirtualMachine`, and
wires it through forms, filtersets, tables, views, the REST API,
GraphQL, navigation, search, documentation, and tests.
Explicit values set on a virtual machine continue to take precedence,
and changes to a type do not retroactively update existing VMs.
2389feea6b