All vbox_job.xml to specify ranges of port forwardings,
and whether the access to the host ports is remote or local.
This is to support MPI for Rytis.

Notes:
- the existing feature (pf_guest_port) is used only for web graphics.
  The host port is always dynamically assigned;
  there's no reason to have <pf_host_port>; I removed it from the doc.
  There's no reason to write the host port to a file; I removed that.
- The new feature is only for fixed host ports.
- The checkpoint file can stay the way it is.