✨ Link Host to VirtualMachine + validate Container image ID

[fanshan]

Host ↔ VirtualMachine relationship

• Host model: add optional OneToOneField to virtualization.VirtualMachine (SET_NULL, related_name="docker_hosts"). One Host may be linked to at most one VirtualMachine; the link is optional on both sides.
• Migration 1044_host_virtual_machine: AddField for the new FK.
• HostTable: add linkified virtual_machine column (available but not in default columns).
• HostSerializer (API): expose virtual_machine via VirtualMachineSerializer.
• HostFilterSet / filtersets.py: virtual_machine_id filter.
• HostView queryset: prefetch_related extended with virtual_machine.
• host.html template: show the linked VM in the Host detail panel.
• template_content.py + virtual_machine_host_table.html: inject a "Docker Hosts" table on the VirtualMachine detail page using NetBox's TemplateExtension mechanism.

Host add / edit form split

• HostAddForm (new, NetBoxModelForm): used exclusively at creation time. Fields: name, endpoint, virtual_machine. Tags are suppressed via __init__ → self.fields.pop("tags", None) so that TagsMixin.__init__ still runs cleanly without a KeyError at line 143 of mixins.py.
• HostForm (existing, NetBoxModelForm): used for editing. Gains virtual_machine field; keeps tags.
• urls.py: hosts/add/ now routes to the new HostAddView.
• HostAddView (new, ObjectEditView): uses HostAddForm, always runs the token-creation logic on POST (no dead not "pk" in url_kwargs guard needed).
• HostEditView: dead alter_object override removed (edit URLs always carry a pk so the creation branch could never fire).

Container validation

• Container.clean(): reject saving a Container whose Image has no ImageID (i.e. the image has not been pulled yet). Error targets the image field: "Image has no ID yet. Pull or refresh it first."

Tests

• test_container_validation.py:
  • test_that_container_image_must_have_an_id: expects ValidationError when the image has no ImageID.
  • test_that_container_image_with_id_passes_validation: clean() succeeds when ImageID is set. Adds image1_pulled fixture (ImageID="sha256:abc123").
• test_host_virtual_machine.py (new):
  • host without VM is valid (default None)
  • host with VM is saved and reloaded correctly
  • OneToOne constraint raises IntegrityError (wrapped in transaction.atomic() savepoint so the test transaction stays intact)
  • SET_NULL on VM deletion nullifies the host FK
  • unlinking sets FK to None without deleting either object
  • reverse accessor vm.docker_host returns the linked Host