Update allowvssprovider in xenstore_data

My previous commit was missing the vm-data part so the original
bug was still present.

The command line fix:
xe vm-param-set uuid={UUID} xenstore-data:vm-data/allowvssprovider=false

This commit adds vm-data to the xenstore message and will properly
disallow vss.

Fixes: bug #1112673

Change-Id: I85ef25a9b61795c98c12549f77cd7ca7305861d2
Merge "Update allowvssprovider in xenstore_data"