virtio-gpu: fix memory leak in resource attach backing

In the resource attach backing function, everytime it will
allocate 'res->iov' thus can leading a memory leak. This
patch avoid this.

Signed-off-by: Li Qiang <liq3ea@gmail.com>
Message-id: 1483003721-65360-1-git-send-email-liq3ea@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
