pygrub/grub: always use integer for default entry

pygrub/grub: always use integer for default entry

The original code set the default to either a string or an integer
(0) and relies on a Python 2 specific behaviour to work (integer is
allowed to be compared to string in Python 2 but not 3).

Always use integer. The caller (pygrub) already has code to handle
that.

Reported-by: M A Young <m.a.young@durham.ac.uk>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>