Fix Packer - QEMU doesn't boot to ISO image.

windows_10.json

@@ -27,6 +27,10 @@
           "-drive",
           "file=windows_10-qemu/{{ .Name }},if=virtio,cache=writeback,discard=ignore,format=qcow2,index=1"
         ],
+        [
+          "-drive",
+          "file={{ user `iso_url` }},media=cdrom,index=2"
+        ],
         [
           "-drive",
           "file={{ user `virtio_win_iso` }},media=cdrom,index=3"