ui: add trace events related to VNC client throttling

ui: add trace events related to VNC client throttling

The VNC client throttling is quite subtle so will benefit from having trace
points available for live debugging.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20171218191228.31018-13-berrange@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
(cherry picked from commit 6aa22a29187e1908f5db738d27c64a9efc8d0bfa)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>