A wrapper that calls g_fsync on Win32/macOS and fdatasync
elsewhere. g_fsync is a stronger flush than we need but it
satisfies the caller's requirements & matches the approach
gnulib takes.
Reviewed-by: NFabiano Fidêncio <fidencio@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>