Add missing execution mode documentation for the 'guest_cpu_enter' and
'guest_cpu_reset' events.

Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
Message-id: 147566900921.7708.656450813307396468.stgit@fimbulvetr.bsc.es
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>