Retrieve disk format and virtual size directly (#1274)

When uploading disks to libvirt storage it is unnecessary to require
that the virtual size or the format be provided as these can be
retrieved by calling qemu-img on the box files to retrieve the required
information.

Update the handle box image support to separate the handling of the two
different formats and remove the need to specify the additional settings
in the case of the V2 format for multi disk boxes.

tools/create_box_with_two_disks.sh

@@ -17,15 +17,12 @@ then
     cat > "${NEW_PATH}/${BOX_VERSION}/libvirt/metadata.json" <<EOF
 {
   "provider": "libvirt",
-  "format": "qcow2",
   "disks" : [
-      {
-          "virtual_size": 2
-      },
-      {
+    {
+    },
+    {
           "path":"disk2.qcow2",
-          "virtual_size":10
-      }
+    }
   ]
 }
 EOF