Remove extra and legacy args from the bootloader invocation

Looks like all XS's pygrub and eliloader do is append them.
Do that anyway following the bootloader execution. This way
we don't depend upon having XS's pygrub at all.

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>
Merge pull request #4 from jonludlam/pygrub_extra_args

Remove extra and legacy args from the bootloader invocation