efi_loader: correctly setup device paths for block devices

According to the UEFI spec the numbering of partitions has to
start with 1.

Partion number 0 is reserved for the optional device path for
the complete block device.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>