When the app destroys the vtest renderer, the vtest client shuts down the socket or directly close it, and the detection of such on the server side is via a read with zero bytes returned without errors. This change adds a non-error result code for such case to avoid confusions.
vkcube launch and close, with vtest server:
$ /usr/local/bin/virgl_test_server --no-fork --multi-clients --venus --no-virgl multi-clients enabled: clients must trust each other
proxy: proxy server with pid 1489737 vtest_client_dispatch_commands: client context created. client failed: VTEST_CLIENT_ERROR_INPUT_READ
proxy: proxy server with pid 1568433 vtest_client_dispatch_commands: client context created. client: VTEST_CLIENT_DISCONNECTED