agent: Report number of CPU cores visible online on the guest OS

This patches a new message and increases the Guest Agent API to
version 1.

The new message has this format:
{"__name__": "number-of-cpus", "count": <int>}

The reported number 'count' is the number of visible online CPU cores
on the guest OS system.

Change-Id: Ief777608519a35319665c4cd0f580b7b38305b3e
Guest-Agent-API-Version: 1
Bug-URL: https://bugzilla.redhat.com/show_bug.cgi?id=1063280
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>