virtio-gpu: add sanity check

Require a minimum 16x16 size for the scanout, to make sure the guest
can't set either width or height to zero.  This (a) doesn't make sense
at all and (b) causes problems in some UI code.  When using spice this
will triggers an assert().
Reported-by: NTyler Slabinski <tslabinski@slabity.net>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20190527091226.4943-1-kraxel@redhat.com

hw/display/virtio-gpu.c

@@ -677,6 +677,8 @@ static void virtio_gpu_set_scanout(VirtIOGPU *g,
 
     if (ss.r.x > res->width ||
         ss.r.y > res->height ||
+        ss.r.width < 16 ||
+        ss.r.height < 16 ||
         ss.r.width > res->width ||
         ss.r.height > res->height ||
         ss.r.x + ss.r.width > res->width ||