qemu_disk_img: Update md5sum_bin to D:\coreutils\md5sum.exe for windows guest.

We put md5sum.exe in win_utils.

Signed-off-by: Feng Yang <fyang@redhat.com>
Merge pull request #1388 from FengYang/qemu_disk_img

qemu_disk_img: Update md5sum_bin to D:\coreutils\md5sum.exe for windows ...