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

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.

PR: 233098
Reviewed by: imp, tsoome
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D36490