bus_space_tag. For now, bus_space_tag's only member is
bst_type, the type of space, which is either X86_BUS_SPACE_IO
or X86_BUS_SPACE_MEM. In the future, new bus_space_tag members
will refer to override-functions installed by a new function,
bus_space_tag_create(9).
Add pointers to constant struct bus_space_tag, x86_bus_space_io and
x86_bus_space_mem. Use them to replace most uses of X86_BUS_SPACE_IO
and X86_BUS_SPACE_MEM.
Add an x86-specific bus_space_is_equal(9) implementation that compares
the two tags' bst_type.
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