Lets the user manage event arguments as a list, and simplifies argument
concatenation.

Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 145641858432.30295.3069911069472672646.stgit@localhost
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>