virtio-gpu: call cleanup mapping function in resource destroy

If the guest destroy the resource before detach banking, the 'iov'
and 'addrs' field in resource is not freed thus leading memory
leak issue. This patch avoid this.

Signed-off-by: Li Qiang <liq3ea@gmail.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 1480386565-10077-1-git-send-email-liq3ea@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
