mark all of 0 -> IOM_BEGIN as used, even though there are some regions
which are left for use by the BIOS (namely, the first 4k page and the
area after biosbasemem*1024 but before IOM_BEGIN). Drivers wishing to
manipulate these areas should map them specially, with _i386_memio_map()
rather than bus_space_map() or i386_memio_map().