Return CDEMU parameters to ebda segment (instead of using globals).

Return CDEMU parameters to ebda segment (instead of using globals).

The cdemu is setup in the boot stage after globals are made
    read-only.  So, globals can't be used.
Also, move hdcount from a global to BDA (it should be in BDA).
Don't attempt to set blksize/sector count in atapi_is_ready() - it's
    not useful.