pygrub/GrubConf: fix boot problem for fedora 19 grub.cfg (2nd attempt)

pygrub/GrubConf: fix boot problem for fedora 19 grub.cfg (2nd attempt)

Booting a fedora 19 domU failed because a it could not properly
parse the grub.cfg file. This was cased by

	set default="${next_entry}"

This statement actually is within an 'if' statement, so maybe it would
be better to skip code within if/fi blocks...
But this patch seems to work fine.

Signed-off-by: Marcel Mol <marcel@mesa.nl>
Acked-by: Ian Campbell <ian.campbell@citix.com>
Tested-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>