When both PARTNAME and ID_PART_ENTRY_NAME are specified, rook takes
PARTNAME in account, overriding ID_PART_ENTRY_NAME. As PARTNAME is not
always updated by the kernel after sgdisk --change-name, it's better to
do the opposite. It also makes GetDevicePartitions coherent with
parsePartLabel (called by GetPartitionLabel).
Signed-off-by: Mikaël Cluseau <mikael.cluseau@gmail.com>