fix(instances): clones roll back their copies; own error messages stay

Clone:
- only shut-off VMs are cloned, as the UI already required: a running
  guest keeps writing while its disks are copied; the definition comes
  from the persistent XML;
- MAC addresses and disk destinations are checked before anything is
  allocated;
- every volume the clone creates (NVRAM and disks) is tracked; if a
  later copy or the definition fails, only those are deleted, and any
  that cannot be deleted are named in the error. Before, a failed clone
  left its earlier copies behind.

libvirtError(msg) reports the thread's last libvirt error instead of
msg when there is one, so WebVirtCloud's own errors could show an
earlier, unrelated failure. util.OperationError, a libvirtError
subclass that keeps its message, replaces those raises in vrtManager,
instances and nwfilters; 'except libvirtError' still catches it.

vrtManager/test_util.py

@@ -128,5 +128,21 @@ class TestVrtManagerUtil(unittest.TestCase):
         self.assertIsNone(vol_dev_type("unknown_bus"))
 
 
+
+class OperationErrorTestCase(unittest.TestCase):
+    def test_keeps_its_own_message_after_a_libvirt_failure(self):
+        import libvirt
+        from vrtManager.util import OperationError
+
+        conn = libvirt.open("test:///default")
+        self.addCleanup(conn.close)
+        with self.assertRaises(libvirt.libvirtError):
+            conn.lookupByName("no-such-domain")  # leaves a thread-local last error
+        self.assertNotEqual(str(libvirt.libvirtError("own message")), "own message")
+        error = OperationError("own message")
+        self.assertEqual(str(error), "own message")
+        self.assertIsInstance(error, libvirt.libvirtError)
+
+
 if __name__ == "__main__":
     unittest.main()