virtio-gpu: fix crashes upon warm reboot with vga mode

virtio-gpu: fix crashes upon warm reboot with vga mode

With vga=775 on the Linux command line a first boot of the VM running
Linux works fine. After a warm reboot it crashes during Linux boot.

Before that, valgrind points out bad memory write to console
surface. The VGA code is not aware that virtio-gpu got a message
surface scanout when the display is disabled. Let's reset VGA graphic
mode when it is the case, so that a new display surface is created
when doing further VGA operations.

https://bugs.launchpad.net/qemu/+bug/1784900/

Reported-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Tested-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Message-id: 20180803153235.4134-1-marcandre.lureau@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>