It used to be the case that you need to create dmabuf for kvmfr devices
to be able to mmap them. But after #457, this is no longer needed.
Directly mmaping the kvmfr device has the advantage of avoiding the
creation of a dmabuf, which has cost (e.g. the list of pages, the
scatterlist, etc.).
These two functions were added in 9ff1859dc134e9f3196703296f868040c77a4dd6
for Windows, but were never used on Linux.
Adding stubs will allow the host to compile on Linux.
These should be fixed later.
This makes it a compile-time error to call a function that semantically
takes no parameters with a nonzero number of arguments.
Previously, such code would still compile, but risk blowing up the stack
if a compiler chose to use something other than caller-cleanup calling
conventions.
