fix(computes): a VM migrates to its host's panel address when it has no migration address

Without a migration address libvirt sends a migrating VM to the
destination's own host name, which the source host often cannot resolve:
a live migration to a host added without one failed ("Unable to resolve
address ..."), also when the other host had one. The destination's
address on the Computes page, which the panel reaches it at, is now the
default; the migration address stays for another network. A host the
panel reaches on its loopback keeps libvirt's default.

instances/tests/test_migrate_job.py

@@ -169,6 +169,26 @@ class MigrateJobTestCase(TransactionTestCase):
             get_kind(op.kind).run(op, ctx)
         self.assertIn("Unable to resolve address 'kvm2.example.com'", public_error_text(raised.exception))
 
+    def test_each_way_goes_to_the_destination_s_migration_address_or_its_panel_address(self):
+        # one host with a migration address, the other without: no name resolution either way
+        self.dest.migration_address = ""
+        self.dest.save()
+        self.source.migration_address = "198.51.100.50"
+        self.source.save()
+        for target, uri, key in ((self.dest, "tcp://192.0.2.51", "key-mig00011"), (self.source, "tcp://198.51.100.50", "key-mig00012")):
+            with self.subTest(target=target.name):
+                self.vm.compute = self.source if target == self.dest else self.dest
+                self.vm.save()
+                self.client.post(reverse("instances:migrate", args=[self.vm.pk]),
+                                 {"compute_id": target.pk, "submission_key": key, "live_migrate": "true"})
+                op = Operation.objects.latest("created_at")
+                ctx = self.start(op)
+                get_kind(op.kind).precheck(op, ctx)
+                get_kind(op.kind).run(op, ctx)
+                self.assertEqual(self.dest_conn.moveto.call_args.kwargs["uri"], uri)
+                Operation.objects.all().delete()
+                Claim.objects.all().delete()
+
     def test_cancel_and_progress_ask_the_source_vm(self):
         op = self.queue(live_migrate="true")
         with patch("instances.jobs.wvmInstance") as proxy: