tools/pygrub: Failing to set value to 0 in Grub2ConfigFile

In Grub2ConfigFile the code to handle ${saved_entry} and ${next_entry}
sets arg = "0" but this now does nothing following c/s d1b93ea261
"tools/pygrub: Make pygrub understand default entry in string format"
which replaced arg.strip() with arg_strip in the following line.  This
patch restores the previous behaviour.

Signed-off-by: Michael Young <m.a.young@durham.ac.uk>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

tools/pygrub/src/GrubConf.py

@@ -440,7 +440,7 @@ class Grub2ConfigFile(_GrubConfigFile):
                     arg_strip = arg.strip()
                     if arg_strip == "${saved_entry}" or arg_strip == "${next_entry}":
                         logging.warning("grub2's saved_entry/next_entry not supported")
-                        arg = "0"
+                        arg_strip = "0"
                     setattr(self, self.commands[com], arg_strip)
                 else:
                     logging.info("Ignored directive %s" %(com,))