This is not a trivial task in parallel, and I would strongly suggest you to use the checkpoint functionality in DOLFIN to save this data to a format supported by DOLFIN (say HDF5File or XDMFFile), which can be read in again in parallel.
See for instance: Loading xdmf data back in - #4 by dokken
Also note that xml formats are not suitable for parallel computation, and you should use the xdmf-file format