ui: fix refresh of VNC server surface

In previous commit

  commit c7628bff
  Author: Gerd Hoffmann <kraxel@redhat.com>
  Date:   Fri Oct 30 12:10:09 2015 +0100

    vnc: only alloc server surface with clients connected

the VNC server was changed so that the 'vd->server' pixman
image was only allocated when a client is connected.

Since then if a client disconnects and then reconnects to
the VNC server all they will see is a black screen until
they do something that triggers a refresh. On a graphical
desktop this is not often noticed since there's many things
going on which cause a refresh. On a plain text console it
is really obvious since nothing refreshes frequently.

The problem is that the VNC server didn't update the guest
dirty bitmap, so still believes its server image is in sync
with the guest contents.

To fix this we must explicitly mark the entire guest desktop
as dirty after re-creating the server surface. Move this
logic into vnc_update_server_surface() so it is guaranteed
to be call in all code paths that re-create the surface
instead of only in vnc_dpy_switch()

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Peter Lieven <pl@kamp.de>
Tested-by: Peter Lieven <pl@kamp.de>
Message-id: 1471365032-18096-1-git-send-email-berrange@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
(cherry picked from commit b69a553b)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>