agent: Add new command setNumberOfCPUs(count)

This adds the support for the new message 'set-number-of-cpus'
and implements it as GuestAgent.setNumberOfCPUs(count) method in the
guest agent.

This shall be called when we hot(un)plug cpus to be able to potentially
trigger behavior on the guest side to online CPU cores.

Change-Id: I330f789eee9e5b7a457bc6defa744c0a6cd6c399
Guest-Agent-API-Version: 1
Bug-Url: https://bugzilla.redhat.com/1063281
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26531
Reviewed-by: Michal Skrivanek <michal.skrivanek@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>