xen: add Activations.dump for debugging

This prints the event counter associated with each event channel.
It's useful to see which channels are 'hot'.
xen: print a message to the console if we fail with Generation.Invalid

This should only happen on resume.
xen: move the event channel query into the OCaml main loop

We now call
  Eventchn.look_for_work
    -- this safely tests the bits in the shared bitmap and updates
       the OCaml array
  Activations.run
    -- this checks the OCaml array and wakes up waiting threads
in the OCaml 'main loop'.