fix screendump

fix screendump

Commit 45efb16124efef51de5157afc31984b5a47700f9 optimized a bit too
much.  We can skip the vga_invalidate_display() in case no console
switch happened because we don't need a full redraw then.  We can *not*
skip vga_hw_update() though, because the screen content will be stale
then in case nobody else calls vga_hw_update().

Trigger: vga textmode with vnc display and no client connected.

Reported-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Alon Levy <alevy@redhat.com>
Tested-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>