Libvirt: Repair metadata injection into guests

The change to use Instance objects when spawning instances
changed the format of the 'metadata' property.  But this wasn't
taken into account in the code which actually injects metadata
into /meta.js in the guest's fs.

This patch ensures that the meta.js file injection expects the
new format.

Closes-bug: #1284733

Change-Id: I37049ac01b65d30d588578e351fcddac457f95a7
Merge "Libvirt: Repair metadata injection into guests"