Adds temporary chown to sparse_copy.

`sparse_copy` needs read and write access to the devices. Since we
cannot shell out to a run-as-root command here, we temporarily take
ownership of the device.

Change-Id: I891c38dbcba7177286dca729684c88ac065bd085

nova/utils.py

@@ -1523,3 +1523,23 @@ def read_file_as_root(file_path):
         return out
     except exception.ProcessExecutionError:
         raise exception.FileNotFound(file_path=file_path)
+
+
+@contextlib.contextmanager
+def temporary_chown(path, owner_uid=None):
+    """Temporarily chown a path.
+
+    :params owner_uid: UID of temporary owner (defaults to current user)
+    """
+    if owner_uid is None:
+        owner_uid = os.getuid()
+
+    orig_uid = os.stat(path).st_uid
+
+    if orig_uid != owner_uid:
+        execute('chown', owner_uid, path, run_as_root=True)
+    try:
+        yield
+    finally:
+        if orig_uid != owner_uid:
+            execute('chown', orig_uid, path, run_as_root=True)