fix(migration): remove the NVRAM file a live migration leaves on the source

libvirt undefines the source domain after a migration but keeps its NVRAM
file. A VM created later on that host with the same name reuses the file
and starts with the old VM's UEFI variables (boot entries, enrolled keys):
checked on libvirt 11.10, where only VIR_DOMAIN_START_RESET_NVRAM
recreates it from the template.

After a successful migration the file is now deleted on the source, as a
volume of the pool on libvirt's NVRAM directory or of a transient one.
Only a file in /var/lib/libvirt/qemu/nvram, and only when the destination
cannot create a marker file the source just created there (an exclusive
create also fails on shared storage whose listing an NFS client caches).
A failed removal is logged with the migration, which stands and is
recorded.

instances/test_migration_log.py

@@ -7,6 +7,7 @@ from computes.models import Compute
 from django.contrib.auth import get_user_model
 from django.test import TestCase
 from django.urls import reverse
+from libvirt import libvirtError
 from logs.models import Logs
 from rest_framework.test import APIClient
 
@@ -22,11 +23,18 @@ class MigrationLogTests(TestCase):
         self.dest = Compute.objects.create(name="ml-dst", hostname="198.51.100.9", login="root", password="", type=2)
         self.vm = Instance.objects.create(compute=self.source, name="vm", uuid="u-ml")
 
-    def migrate(self, outcome, **request):
+    def migrate(self, outcome, nvram=None, remove_nvram=True, **request):
+        self.proxy = MagicMock()
+        self.proxy.get_autostart.return_value = False
+        self.proxy.get_nvram.return_value = nvram
+        if isinstance(remove_nvram, Exception):
+            self.proxy.remove_nvram.side_effect = remove_nvram
+        else:
+            self.proxy.remove_nvram.return_value = remove_nvram
         with patch.object(Compute, "status", True), \
              patch("instances.utils.wvmInstances") as instances, \
              patch("instances.utils.wvmInstance"), \
-             patch.object(Instance, "proxy", MagicMock()), \
+             patch.object(Instance, "proxy", self.proxy), \
              patch.object(Instance, "autostart", False):
             if isinstance(outcome, Exception):
                 instances.return_value.moveto.side_effect = outcome
@@ -34,6 +42,40 @@ class MigrationLogTests(TestCase):
                 instances.return_value.moveto.return_value = outcome
             return migrate_instance(self.dest, self.vm, self.admin, **request)
 
+    def test_the_nvram_file_left_on_the_source_is_removed(self):
+        self.migrate("live", nvram="/var/lib/libvirt/qemu/nvram/vm_VARS.fd", live=True)
+        path, destination = self.proxy.remove_nvram.call_args[0]
+        self.assertEqual(path, "/var/lib/libvirt/qemu/nvram/vm_VARS.fd")
+        self.assertIsNotNone(destination)
+        self.assertEqual(Logs.objects.get().message, "Instance is migrated(live) to 198.51.100.9")
+
+    def test_a_vm_without_nvram_removes_nothing(self):
+        self.migrate("live", live=True)
+        self.assertFalse(self.proxy.remove_nvram.called)
+
+    def test_a_failed_nvram_removal_is_logged_but_the_migration_stands(self):
+        self.migrate("live", nvram="/var/lib/libvirt/qemu/nvram/vm_VARS.fd",
+                     remove_nvram=libvirtError("cannot delete"), live=True)
+        self.assertEqual(
+            Logs.objects.get().message,
+            "Instance is migrated(live) to 198.51.100.9; removing its NVRAM file "
+            "/var/lib/libvirt/qemu/nvram/vm_VARS.fd on the source failed: cannot delete",
+        )
+        self.vm.refresh_from_db()
+        self.assertEqual(self.vm.compute, self.dest)
+
+    def test_any_nvram_removal_error_still_records_the_migration(self):
+        self.migrate("live", nvram="/var/lib/libvirt/qemu/nvram/vm_VARS.fd",
+                     remove_nvram=AttributeError("unexpected"), live=True)
+        self.vm.refresh_from_db()
+        self.assertEqual(self.vm.compute, self.dest)
+        self.assertIn("on the source failed: unexpected", Logs.objects.get().message)
+
+    def test_a_failed_migration_removes_nothing(self):
+        with self.assertRaises(util.OperationError):
+            self.migrate(util.OperationError("no"), nvram="/var/lib/libvirt/qemu/nvram/vm_VARS.fd", live=True)
+        self.assertFalse(self.proxy.remove_nvram.called)
+
     def test_success_names_the_mode_used(self):
         self.migrate("non-live, compressed", live=False, compress=True)
         log = Logs.objects.get()