python: Let RHashtable be returned as a Python dict.

The initial proposal was suggested by Matt Booth and discussed on the
mailing list here:

https://www.redhat.com/archives/libguestfs/2013-April/msg00007.html

python/examples/guestfs-python.pod

@@ -7,7 +7,7 @@ guestfs-python - How to use libguestfs from Python
 =head1 SYNOPSIS
 
  import guestfs
- g = guestfs.GuestFS ()
+ g = guestfs.GuestFS (python_return_dict=True)
  g.add_drive_opts ("disk.img", format="raw", readonly=1)
  g.launch ()
 
@@ -18,6 +18,17 @@ programming language.  This page just documents the differences from
 the C API and gives some examples.  If you are not familiar with using
 libguestfs, you also need to read L<guestfs(3)>.
 
+=head2 python_return_dict=True
+
+All new code should construct the handle using:
+
+ g = guestfs.GuestFS (python_return_dict=True)
+
+This indicates that your program wants to receive Python dicts for
+methods in the API that return hashtables.
+
+In a future version of libguestfs, this will become the default.
+
 =head2 EXCEPTIONS
 
 Errors from libguestfs functions are mapped into C<RuntimeException>