[nova][oot] Adding `context` to ComputeDriver.unrescue

Though still very much WIP, emulated TPM [1] is looking like it will need to add the RequestContext to the ``unrescue`` ComputeDriver method [2]. This is a heads up to out-of-tree virt driver maintainers to keep an eye on this patch, as you will need to update your overrides accordingly once it merges. Thanks, efried [1] https://review.opendev.org/#/c/631363/ [2] https://review.opendev.org/#/c/631363/31/nova/virt/driver.py