MFC r305036:
Some versions of SLOF do not append the partition number to the boot
device argument to the stage-1 bootloader. In such cases, boot1 would
only try to read the entire device rather than checking for partitions.
Instead of panic'ing, fall back to reading the partitions as normal in
such situations. This was preventing boot of installed systems on some
versions of PowerKVM.

PR: kern/211599