client.file_io failing on win10: dr_safe_write() not returning partial size written due to NtWriteVirtualMemory behavior change

The client.file_io test is failing on win10:

116: ERROR in overlap dr_safe_write() 0x254c0000 0x254c2000 -1

NtWriteVirtualMemory is correctly writing the expected partial up to the page boundary, and returning STATUS_PARTIAL_COPY as expected.

But in ReturnLength it's returning -1! Must be a kernel behavior change as pre-win10 kernels returned the actual partial number of bytes written.