Add the core of the instance status collector

Add the core of the instance status collector

Add the Xen instance status data collector with only its core features.
The next commits will add more reporting functionalities.

The access to the collector is made possible through the mon-collector
tool.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>