monitor: Reduce casting of QAPI event QDict

Make the variables holding the event QDict instead of QObject.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1444921716-9511-2-git-send-email-armbru@redhat.com>