Allow tracing of the operation of visitors

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1475246744-29302-4-git-send-email-berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[visit_type_uint8() & friends rearranged slightly for clarity]
Signed-off-by: Markus Armbruster <armbru@redhat.com>