grub2: fix a few minor issues in help text

The help text of grub2 explains the detailed steps to create a disk
image with grub2 installed on it. However, the steps for the
BIOS-based systems have a few minor issues fixed by this patch:

 - When calling partx to get the partitions detected, we should do it
   on the /dev/loop0 block device, and not on the underlying disk.img
   image file.
 - The grub-bios-setup utility must be called as root to work properly
   on /dev/loop0.
 - The steps to cleanup the partx and loop device were missing.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>