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>

tools/pygrub/src/GrubConf.py

@@ -233,7 +233,11 @@ class _GrubConfigFile(object):
         if val == "saved":
             self._default = 0
         else:
-            self._default = val
+            try:
+                self._default = int(val)
+            except ValueError:
+                logging.warning("Invalid value %s, setting default to 0" %(val,))
+                self._default = 0
 
         if self._default < 0:
             raise ValueError("default must be non-negative number")