filebackend: Check for old alias in FileBackendMultiWrite
When the config contains the old class name, the change to the new
namespaced class in 1b29f07440f2c8fd039987feb1a075b9d683c841 breaks
existing configs.
Follow-Up: I73fb416573f5af600e529d224b5beb5d2e3d27d3
Change-Id: I5236609dbef5c28c4d220680c501b5813f940e8f