efi_loader: Fix disk dp's for pre-DM/legacy devices

This fixes an issue with OpenBSD's bootloader, and I think should also
fix a similar issue with grub2 on legacy devices.  In the legacy case
we were creating disk objects for the partitions, but not also the
parent device.

Reported-by: Jonathan Gray <jsg@jsg.id.au>
Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>