qxl: save qemu_create_displaysurface_from result

Spotted by Coverity.

https://bugzilla.redhat.com/show_bug.cgi?id=885644

Cc: qemu-stable@nongnu.org
Reported-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
