loader: create separate lists for fd, cd and hd, merge bioscd with biosdisk

Create unified block IO implementation in BIOS version, like it is done in UEFI
side. Implement fd, disk and cd device lists, this will split floppy devices
from disks and will allow us to have consistent, predictable device naming
(modulo BIOS issues).

Differential Revision: https://reviews.freebsd.org/D17888