virtcontainers: GetOOMEvent make agent's goroutines leak

[liubin]

Description of problem

Running the latest kata for some while, goroutines in agent will increasing continuously, about 1 goroutine per minute.

And it also produced many too many logs:

May 21 09:36:00 master kata[30282]: time="2020-05-21T09:36:00.669881517Z" level=warning msg="failed to get OOM event from sandbox" error="rpc error: code = DeadlineExceeded desc = context deadline exceeded" sandbox=866635a11e6c426bf4ddf57c8d05ce9fa894e00eb7ae6b8ac9fb3546a07cf86b

Expected result

Goroutines in agent should keep in a stable number.

Reason

GetOOMEvent rpc call used 60 seconds as timeout:

https://github.com/kata-containers/runtime/blob/master/virtcontainers/kata_agent.go#L2015

And code in agent will blocked at https://github.com/kata-containers/agent/blob/master/grpc.go#L1807