xenapi: ensure instance metadata always injected into xenstore

It is possible to use the agent, cloud-init and config drive
together on a single server. In this case we should still
be injecting the instance metadata.

The current code is inconsistent between spawn and migrations.

The correct approach is to always inject the data into xenstore.

fixes bug 1188618

Change-Id: I253b34234544cff353985a5b53ec084ea332535a
Merge "xenapi: ensure instance metadata always injected into xenstore"