libxl: store a copy of configuration when creating domain

libxl: store a copy of configuration when creating domain

The configuration is stored in libxl-json format. It will be used as
template to reconstruct domain configuration during runtime.

There's only one write to disk when domain creation finishes. We
therefore have a window that the domain exists but has no JSON config in
disk. We define this state as domain being created or destroyed. Any
other operations that need to access JSON config should bail.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>