fix(instances): a failed VM creation leaves no volumes behind

Web form: the cache mode and the UEFI firmware value were checked only
after the disk volume had been created; an invalid one answered 500 and
left the volume. All checks now run before any allocation. Volumes the
request creates are removed on any failure up to the definition; ones
that cannot be removed are reported. After the definition the VM uses
them, so a later failure keeps them.

API: the new volume was never removed when the definition failed, and
the error was a 500. It is removed now and a libvirt error returns 400.

instances/test_create_rollback.py

@@ -0,0 +1,48 @@
+from unittest.mock import patch
+
+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
+
+FORM = {
+    "name": "rb-vm", "firmware": "BIOS", "vcpu": 1, "vcpu_mode": "host-model", "memory": 128,
+    "hdd_size": 1, "storage": "default", "mac": "52:54:00:aa:04:01", "networks": "default",
+    "nwfilter": "", "net_model": "virtio", "cache_mode": "none", "video": "vga",
+    "listener_addr": "0.0.0.0", "console_pass": "", "add_cdrom": "None", "add_input": "None",
+    "create": True,
+}
+
+
+class CreateRollbackTestCase(TestCase):
+    """R-08: volumes are removed only while the VM is not defined yet."""
+
+    def setUp(self):
+        admin = get_user_model().objects.create_superuser("rb_admin", "r@example.com", "pw")
+        self.client.force_login(admin)
+        self.client.raise_request_exception = False
+        self.compute = Compute.objects.create(name="rb", hostname="127.0.0.1", login="", password="", type=4)
+
+    def post(self, **conn_setup):
+        with patch("instances.views.wvmCreate") as wvm:
+            conn = wvm.return_value
+            conn.get_cache_modes.return_value = {"none": "Disable cache"}
+            conn.get_instances.return_value = []
+            conn.get_storages.return_value = ["default"]
+            conn.get_networks.return_value = ["default"]
+            conn.create_volume.return_value = "/pool/rb-vm.qcow2"
+            for attr, value in conn_setup.items():
+                getattr(conn, attr).side_effect = value
+            self.client.post(reverse("instances:create_instance", args=[self.compute.id, "x86_64", "q35"]), FORM)
+        return conn
+
+    def test_failed_definition_removes_the_new_volume(self):
+        conn = self.post(create_instance=libvirtError("rejected"))
+        conn.delete_volume.assert_called_once_with("/pool/rb-vm.qcow2")
+
+    def test_failure_after_the_definition_keeps_the_volume(self):
+        with patch("instances.views.Instance.objects.get_or_create", side_effect=RuntimeError("db down")):
+            conn = self.post()
+        conn.create_instance.assert_called_once()
+        conn.delete_volume.assert_not_called()