MFC r342151, r342161:
loader: zfs reader should not probe partitionless disks

First of all, normal setups can not boot such pools as the tools
do not support installing boot programs.

Secondly, for proper pool configuration detection, we need to checks all
four label copies on disk, 2 from front and 2 from the end of the disk,
but zfs label does not contain the size of the disk - so we depend on
firmware to report the correct disk size or use information from the
partition table.

Without partition table, we only can rely on firmware to report and support
disk IO properly.

Relnotes: yes