pygrub: Add option to list grub entries

pygrub: Add option to list grub entries

The argument to "--entry" allows 2 syntaxes, either directly the entry
number in menu.lst, or the whole string behind the "title" key word.
This poses the following issue:

From Dom0 there is no way to guess the number and, or the complete
title string because this string contains the kernel version, which
will change with a kernel update.

This patch adds [-l|--list-entries] as an argument to pygrub.

Signed-off-by: Charles Arnold <carnold@suse.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>