test: the computes tests share one fake libvirt domain

test_ownership, test_ownership_race and test_reconciliation each defined
the same fake domain (a name and a UUID); it is computes/tests/fakes.py.

computes/tests/fakes.py

@@ -0,0 +1,14 @@
+"""Test doubles for the computes tests."""
+
+
+class Domain:
+    """A libvirt domain as the VM list reads it: its name and UUID."""
+
+    def __init__(self, name, uuid):
+        self._name, self._uuid = name, uuid
+
+    def name(self):
+        return self._name
+
+    def UUIDString(self):
+        return self._uuid