Option --rw-blockdev: ensure to set device to rw between grub-install runs + don...

Option --rw-blockdev: ensure to set device to rw between grub-install runs + don't set rw on source

If we're invoking set_rw on the source device this probably
points to the source ISO which will fail.

On the other side we invoke grub-intall with and without --force
in a loop and have to unblock the underlying block device in
between as well, otherwise failing to install.