stand: i386: take into account disk sector size for blk calculation

[kevans]

Summary

disk_blocks assumes BIOSDISK_SECSIZE, but the media may not be using
it. In particular, bioscd on Parallels presents a 2K sector size, so
we end up with a short disk_blocks and subsequent validation fails when
trying to read /boot/lua.

[kevans]

Simplify, just divide; bd_sectorsize should always be a multiple of 512.

[tsoome]

Thanks!