Different APEI tables can reuse the same registers (and sometimes
different views of the same register, e.g. 32- vs 64-bit mappings of
the same register). To enable this sharing, apei0 now acts as a bus
device managing a pool of allocated resources and handing out mappings
to child devices which handle individual tables.

Most of the previous apei(4) driver has been moved into a new
hest0 device that is a child of apei0.

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