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/templates/virtualization/virtualmachinetype.html

@@ -0,0 +1,10 @@
+{% extends 'generic/object.html' %}
+{% load i18n %}
+
+{% block extra_controls %}
+  {% if perms.virtualization.add_virtualmachine %}
+    <a href="{% url 'virtualization:virtualmachine_add' %}?virtual_machine_type={{ object.pk }}" class="btn btn-primary">
+      <span class="mdi mdi-plus-thick" aria-hidden="true"></span> {% trans "Add Virtual Machine" %}
+    </a>
+  {% endif %}
+{% endblock extra_controls %}