xenapi: Uninstall instances

The devstack script sometimes failed to clean up the left over
instances. This patch will utilize the xe vm-uninstall force=true
command to get rid of the instances.

Change-Id: I3450e2d5c9a4b1fe8ede39c4cc5c337cd541ea35
diff --git a/tools/xen/install_os_domU.sh b/tools/xen/install_os_domU.sh
index 6eb3013..43247fc 100755
--- a/tools/xen/install_os_domU.sh
+++ b/tools/xen/install_os_domU.sh
@@ -138,9 +138,7 @@
     # Destroy any instances that were launched
     for uuid in `xe vm-list | grep -1 instance | grep uuid | sed "s/.*\: //g"`; do
         echo "Shutting down nova instance $uuid"
-        xe vm-unpause uuid=$uuid || true
-        xe vm-shutdown uuid=$uuid || true
-        xe vm-destroy uuid=$uuid
+        xe vm-uninstall uuid=$uuid force=true
     done
 
     # Destroy orphaned vdis