Fix race conditions with xenstore

xenapi code updates xenstore using _add_to_param_xenstore(), which has
to ensure a key doesn't exist before it can add the same key.  However,
if you have 2 calls at the same time, this is racey.

Fixes bug 1190026

Change-Id: I545b7a9fab0bc4c3749bc75387ed3f6ff27a512f
Merge "Fix race conditions with xenstore"