ui: avoid pointless VNC updates if framebuffer isn't dirty
    
    The vnc_update_client() method checks the 'has_dirty' flag to see if there are
    dirty regions that are pending to send to the client. Regardless of this flag,
    if a forced update is requested, updates must be sent. For unknown reasons
    though, the code also tries to sent updates if audio capture is enabled. This
    makes no sense as audio capture state does not impact framebuffer contents, so
    this check is removed.
    
    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: 20171218191228.31018-5-berrange@redhat.com
    Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
    (cherry picked from commit 3541b08475d51bddf8aded36576a0ff5a547a978)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>