feat(virtualization): Add Virtual Machine Type model

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.

netbox/virtualization/apps.py

@@ -9,10 +9,10 @@ class VirtualizationConfig(AppConfig):
         from utilities.counters import connect_counters
 
         from . import search, signals  # noqa: F401
-        from .models import VirtualMachine
+        from .models import VirtualMachine, VirtualMachineType
 
         # Register models
         register_models(*self.get_models())
 
         # Register counters
-        connect_counters(VirtualMachine)
+        connect_counters(VirtualMachine, VirtualMachineType)