efi_loader: Fix partition offsets

Commit 884bcf6f (efi_loader: use proper device-paths for partitions) tried
to introduce the el torito scheme to all partition table types: Spawn
individual disk objects for each partition on a disk.

Unfortunately, that code ended up creating partitions with offset=0 which meant
that anyone accessing these objects gets data from the raw block device instead
of the partition.

Furthermore, all the el torito logic to spawn devices for partitions was
duplicated. So let's merge the two code paths and give partition disk objects
good offsets to work from, so that payloads can actually make use of them.

Fixes: 884bcf6f (efi_loader: use proper device-paths for partitions)
Reported-by: Yousaf Kaukab <yousaf.kaukab@suse.com>
Signed-off-by: Alexander Graf <agraf@suse.de>