Fix retirement of fences when using async callback
Once support for both async fencing and Venus context are enabled in Qemu, the debug assert in Venus codepath catches attempt to perform a synced retirement of the Venus fences when Qemu polls status of vrend queries.
The retirement shall be skipped for all context types if async cb is used. Previously this stayed unnoticed because Qemu didn't support Venus and crosvm uses fd event instead of doing a periodic poll like Qemu does it. In order to reproduce this issue guest should create both vrend and Venus contexts and then host should perform the polling. In particular this happens in case of running Steam on Qemu with enabled Venus support.
Skip retiring fences if using async callback to fix triggering of the debug check in proxy_context_retire_fences().