trace: push reading of events up a level to tracetool main

Move the reading of events out of the 'tracetool.generate'
method and into tracetool.main, so that the latter is not
tied to generating from a single source of events.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Lluís Vilanova <vilanova@ac.upc.edu>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 1475588159-30598-19-git-send-email-berrange@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>