ploop: specify hypervisor_type=vz property for ploop images

This is necessary to make it possible to filter out compute nodes,
which don't support such type of images.

Change-Id: I347953876e2057e6f3dca71c2f5e8b638b85aaf8
diff --git a/functions b/functions
index 46a7d41..50b72eb 100644
--- a/functions
+++ b/functions
@@ -251,6 +251,7 @@
             image create \
             "$image_name" --public \
             --container-format=bare --disk-format=ploop \
+            --property hypervisor_type=vz \
             --property vm_mode=$vm_mode < "${image}"
         return
     fi