Create /boot/grub/menu.lst for Fedora-18 images

I've tried creating F18 images and ran into this:

  File "/usr/lib/python2.6/site-packages/imagefactory_plugins/EC2Cloud/EC2Cloud.py", line 152, in builder_did_create_target_image
    self.ec2_modify_filesystem()
  File "/usr/lib/python2.6/site-packages/imagefactory_plugins/EC2Cloud/EC2Cloud.py", line 457, in ec2_modify_filesystem
    g.write("/boot/grub/menu.lst", tmpl)
  File "/usr/lib/python2.6/site-packages/guestfs.py", line 5089, in write
    return libguestfsmod.write (self._o, path, content)
RuntimeError: write: open: /boot/grub/menu.lst: No such file or directory

My tiny patch is intended to fix this small issue.