Test flake in credit_update_sent_after_flushing_half_buffer

[papertigers]

As seen in this test run we have a vsock test failure:

	test vsock::poller::test::credit_update_sent_after_flushing_half_buffer ... FAILED
	
	failures:
	
	---- vsock::poller::test::credit_update_sent_after_flushing_half_buffer stdout ----
	
	thread 'vsock::poller::test::credit_update_sent_after_flushing_half_buffer' (189) panicked at lib/propolis/src/vsock/poller.rs:1503:39:
	called `Result::unwrap()` on an `Err` value: Os { code: 11, kind: WouldBlock, message: "Resource temporarily unavailable" }
	
	thread 'vsock-event-loop' (215) panicked at lib/propolis/src/hw/virtio/vsock.rs:127:45:
	mem access for write
	
	
	failures:
	    vsock::poller::test::credit_update_sent_after_flushing_half_buffer
	
	test result: FAILED. 193 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 5.73s