Make grub2 based images work in EC2

 - properly setup grub2 for EC2 usage [bnc #887801]
  + PV-GRUB looks for the grub2 modules in /boot/grub2/xen-x86_64, although
     this is not a "real grub2 target. Symlinking to i386-pc does the trick.
     Using the prefix setting in the grub configuration trips up PV-GRUB.