Summary: Move Xen vmdata to new, non-ssh method

Detail: KVM recently got a patch that did away with a few dozen ssh calls
when programming virtual router (CLOUDSTACK-3163), saving several seconds
for each vm served by the virtual router when the router is rebooted. This
patch updates Xen to use the same method, and cleans up the old script refs.

Reviewed-by: Sheng Yang, Prasanna Santhanam