HYPERV isn't available on all architectures, but just on by default
for i386/amd64. Rather, it only works on i386/amd64 and should only be
built there. Rather than change the default based on which
architecutre, do things more directly by only building it on
i386/amd64 and having it always on. This is how we handle other
options that are relevant only for a few architectures.