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.
Reviewed by: imp, tsoome
(cherry picked from commit 72291cee075d9ef1a608b2a67762b44e024cc0d4)