* Change the Assist API. Rather than passing callbacks in each call, the
callbacks are now registered beforehand. Then change the I/O Assist to
fetch MMIO data via the Mem callback. This allows a guest to perform an
I/O string operation on a memory that is itself an MMIO.
* Introduce two new functions internal to libnvmm, read_guest_memory and
write_guest_memory. They can handle mapped memory, MMIO memory and
cross-page transactions.
* Allow nvmm_gva_to_gpa and nvmm_gpa_to_hva to take non-page-aligned
addresses. This simplifies a lot of things.
* Support the MOVS instruction, and add a test for it. This instruction
is special, in that it takes two implicit memory operands. In
particular, it means that the two buffers can both be in MMIO memory,
and we handle this case.
* Fix gross copy-pasto in nvmm_hva_unmap. Also fix a few things here and
there.
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