pygrub: add compatibility with upstream pygrub

We only supply the --extra_args and --default_args if they are not ""
  -- these haven't been used since the upgrade to pygrub in XenEnterprise
     and this code could probably just be deleted altogether.

We only supply the --vm parameter if we are running eliloader
  -- eliloader needs this to fiddle with the VM state. pygrub doesn't
     need any VM information at all.

fixes xen-org/xenopsd#5