console: skip same-size resize

virtio-gpu does a set-scanout at each frame (it might be a driver
regression). qemu_console_resize() recreate a surface even if the size
didn't change, and this shows up in profiling reports because the
surface is cleared. With this patch, I get a +15-20% glmark2
improvement.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20160826094711.14470-1-marcandre.lureau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>