ui: introduce enum to track VNC client framebuffer update request state

ui: introduce enum to track VNC client framebuffer update request state

Currently the VNC servers tracks whether a client has requested an incremental
or forced update with two boolean flags. There are only really 3 distinct
states to track, so create an enum to more accurately reflect permitted states.

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: 20171218191228.31018-7-berrange@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
(cherry picked from commit fef1bbadfb2c3027208eb3d14b43e1bdb51166ca)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>