tools/pygrub: Make pygrub understand default entry in string format

tools/pygrub: Make pygrub understand default entry in string format

Currently pygrub can only correctly parse grub2's default attribute when it is
specified as a number. If it is set to ${saved_entry} or ${next_entry} then
the first image (i.e. entry number 0) is selected. If any other value is
specified (typically this would be the string in menuentry) pygrub will crash.

This patch will allow pygrub to interpret default attribute if it is specified
as a string (note that in case of submenus only the leaf string will be
considered).

Also issue a warning if default is set to ${saved_entry} or ${next_entry}.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
(cherry picked from commit d1b93ea2615bd789ee28901f1f1c05ffb319cb61)