libxl: config parser: print warning for apparent arbitrary python

libxl: config parser: print warning for apparent arbitrary python

The characters - + . ( ) : are not legal in xl config files but are
valid Python and use of at least one of them is almost essential for
writing arbitrary Python in the config file.

So if we see one of these during lexing, note it, and then after the
parse is complete if it failed we print a special extra warning.

Currently this warning refers to the nonexistent wiki page
  http://wiki.xen.org/xenwiki/PythonInXlConfig
which will have to be written (and/or given a better name) before the
actual 4.1 release.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>