ui/vnc: fix vmware VGA incompatiblities

this fixes invalid rectangle updates observed after commit 12b316d
with the vmware VGA driver. The issues occured because the server
and client surface update seems to be out of sync at some points
and the max width of the surface is not dividable by
VNC_DIRTY_BITS_PER_PIXEL (16).

Reported-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Merge remote-tracking branch 'remotes/kraxel/tags/pull-vnc-2' into staging

vnc: fix vmware VGA incompatiblities

# gpg: Signature made Tue 18 Mar 2014 07:23:10 GMT using RSA key ID D3E87138
# gpg: Good signature from "Gerd Hoffmann (work) <kraxel@redhat.com>"
# gpg:                 aka "Gerd Hoffmann <gerd@kraxel.org>"
# gpg:                 aka "Gerd Hoffmann (private) <kraxel@gmail.com>"

* remotes/kraxel/tags/pull-vnc-2:
  ui/vnc: fix vmware VGA incompatiblities

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>