multiboot header has 0 in mem_upper field

When booting a multiboot image,. mem_upper is now always zero.

To test, build qemu from current git head, then do
  cd tests/multiboot
  ./run_test.sh

You will see the test fail.  In each case mem_upper is 0k.

git-bisect says the bad commit is 0169c511554cb0014a00290b0d3d26c31a49818f in qemu.git