When processing monitor config from guest store head0 width and height
for single-head configurations.  Use these when creating the
DisplaySurface in the local renderer.

This fixes a rendering issue with wayland.  Wayland rounds up the
framebuffer width and height to a multiple of 64, so with odd
resolutions (800x600 for example) the framebuffer is larger than the
actual screen.  The monitor config has the actual screen size though.

This fixes guest display for anything using the local renderer
(non-spice UI, screendump monitor command).
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20180919103057.9666-1-kraxel@redhat.com