fix(instances): a refused XML definition comes back in its editor

On the machine step, "Or define it from XML" lost the definition when it
was refused: a taken name re-rendered the page with an empty, collapsed
editor; malformed XML went on to libvirt, and libvirt's refusal sent the
browser back to an empty page with the reason at the top.

All three refusals now render the page with the section open, the
submitted XML in the editor (and in the field it posts, so it can be sent
again unchanged), and the reason above the editor, which takes the focus.

Fixes #174

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

instances/tests/test_create_form_input.py

@@ -5,6 +5,9 @@ import json
 import uuid
 from unittest.mock import patch
 
+import libvirt
+from computes.models import Compute
+from django.contrib.auth import get_user_model
 from django.test import TestCase
 from django.urls import reverse
 from instances.models import Flavor
@@ -262,3 +265,32 @@ class SubmittedListsTests(TestCase):
     def test_only_a_refused_form_sends_lists(self):
         _, response = render_create_page(self)
         self.assertIsNone(self.lists(response))
+
+
+class XmlRefusalTests(TestCase):
+    """A refused XML definition comes back in its open editor, its reason
+    beside it."""
+
+    def post(self, xml):
+        conn = libvirt.open("test:///default")
+        self.addCleanup(conn.close)
+        compute, _ = Compute.objects.get_or_create(name="testdrv", hostname="localhost", login="", password="", type=4)
+        self.client.force_login(get_user_model().objects.create_superuser("xml", "xml@example.com", "pw"))
+        with patch("vrtManager.connection.connection_manager.get_connection", return_value=conn):
+            response = self.client.post(reverse("instances:create_instance_select_type", args=[compute.id]),
+                                        {"create_xml": "", "dom_xml": xml})
+        self.assertEqual(response.status_code, 200)
+        page = html.fromstring(response.content)
+        self.assertIn("show", page.get_element_by_id("from-xml").classes)
+        self.assertEqual(page.xpath("//*[@id='editor']/text()"), [xml])
+        self.assertEqual(page.xpath("//input[@name='dom_xml']/@value"), [xml])
+        return page.xpath("//*[@id='from-xml']//*[contains(@class, 'form-errors')]")[0].text_content()
+
+    def test_a_taken_name(self):
+        self.assertIn("already exists", self.post("<domain type='test'><name>test</name></domain>"))
+
+    def test_malformed_xml(self):
+        self.assertIn("not well-formed", self.post("<domain type='test'><name>new</name>"))
+
+    def test_a_definition_libvirt_refuses(self):
+        self.assertIn("libvirt", self.post("<domain type='nope'><name>new</name><memory>1024</memory></domain>"))