Pass libvirt name to API via system metadata

When cells does an instance update broadcast upwards, add the
libvirt instance name to the instance's system metadata.

When cells is enabled, use this in the API implementation as the source
of instance_name and display nothing if it's missing, rather than
something misleading.

Change-Id: Id45768eaa5394f8d0f8ced16cdd285f66568dd7d