python: Use bytes instead of str for event callback buffer

The event callback gets a buffer parameter which is usually something
like a log message.  However as it comes from C it is not necessarily
well-formed (eg) UTF-8 but could contain any old sequence of bytes.

In the test case provided by the reporter, we failed to encode the
buffer as 'str' with this error:

  UnicodeDecodeError: 'utf-8' codec can't decode byte 0xc3 in position 137: unexpected end of data

Use 'bytes' instead.  Strictly speaking this changes the type
signature of the callbacks, but our existing Python tests which just
print the buffer using '%s' don't fail and in any case we don't
guarantee the stability of non-C APIs.

Reported-by: Yonatan Shtarkman
See: https://listman.redhat.com/archives/libguestfs/2023-February/030653.html

python/handle.c

@@ -131,7 +131,7 @@ guestfs_int_py_event_callback_wrapper (guestfs_h *g,
   }
 
   /* XXX As with Perl we don't pass the guestfs_h handle here. */
-  args = Py_BuildValue ("(Kis#O)",
+  args = Py_BuildValue ("(Kiy#O)",
                         (unsigned PY_LONG_LONG) event, event_handle,
                         buf, buf_len, py_array);
   if (args == NULL) {