fix: address appliance v8 install review findings

- install: resolve the image directory from the version's settings type and
  skip image handling for docker appliances; guard appliance.images
- appliance schema: validate template_properties against template_type,
  align cpu_throttling with the qemu template, add kvm and version idlepc
- conversion: map IOU image to path, kvm disable to accel=tcg, inherit only
  same-type default settings, symbol fallback from the effective category,
  template_properties cannot override structural fields
- allow clearing netmiko_device_type with an empty string
- download the template symbol regardless of the level it is defined at and
  give qemu guests a default symbol

tests/api/routes/controller/test_templates.py

@@ -166,6 +166,13 @@ class TestTemplateRoutes:
         assert response.json()["name"] == "VPCS_TEST_RENAMED"
         assert response.json()["netmiko_device_type"] == "cisco_ios"
 
+        # the field can also be cleared with an empty string
+        params = {"netmiko_device_type": ""}
+        response = await client.put(app.url_path_for("update_template", template_id=template_id), json=params)
+
+        assert response.status_code == status.HTTP_200_OK
+        assert response.json()["netmiko_device_type"] == ""
+
     async def test_template_delete(self, app: FastAPI, client: AsyncClient) -> None:
 
         template_id = str(uuid.uuid4())