Allow multicore VMs, and take this into account in the sharing

Allow multicore VMs, and take this into account in the sharing

This does not break any test, because it's not well tested yet. Yup,
the ChangeLog is a bit overselling so far. A new test torturing this
to make sure we get the right result in many sharing situation is
still to be written. examples/msg/cloud-simple will be taken as a
basis for that.