ui/vnc: fix potential memory corruption issues

this patch makes the VNC server work correctly if the
server surface and the guest surface have different sizes.

Basically the server surface is adjusted to not exceed VNC_MAX_WIDTH
x VNC_MAX_HEIGHT and additionally the width is rounded up to multiple of
VNC_DIRTY_PIXELS_PER_BIT.

If we have a resolution whose width is not dividable by VNC_DIRTY_PIXELS_PER_BIT
we now get a small black bar on the right of the screen.

If the surface is too big to fit the limits only the upper left area is shown.

On top of that this fixes 2 memory corruption issues:

The first was actually discovered during playing
around with a Windows 7 vServer. During resolution
change in Windows 7 it happens sometimes that Windows
changes to an intermediate resolution where
server_stride % cmp_bytes != 0 (in vnc_refresh_server_surface).
This happens only if width % VNC_DIRTY_PIXELS_PER_BIT != 0.

The second is a theoretical issue, but is maybe exploitable
by the guest. If for some reason the guest surface size is bigger
than VNC_MAX_WIDTH x VNC_MAX_HEIGHT we end up in severe corruption since
this limit is nowhere enforced.

Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
