Do not upload vhdx images as vhd

This change ensures that when uploading vhdx images, we use the
proper format.

At the moment, vhdx images are uploaded as vhd, which can be
troublesome: first because this is misleading, second because the
actual image format may be checked, having the image rejected.

Change-Id: I9578be41ea9dc252404b7553679ac527e08a0ff6

functions

@@ -323,7 +323,7 @@ function upload_image {
         *.vhd|*.vhdx|*.vhd.gz|*.vhdx.gz)
             local extension="${image_fname#*.}"
             image_name=$(basename "$image" ".$extension")
-            disk_format=vhd
+            disk_format=$(echo $image_fname | grep -oP '(?<=\.)vhdx?(?=\.|$)')
             container_format=bare
             if [ "${image_fname##*.}" == "gz" ]; then
                 unpack=zcat