cli test sockets::a_libqtest_session fails now and then with a broken pipe #170

While checking the arm virt PCIe PR on server2, sockets::a_libqtest_session in crates/cli/tests/system.rs failed once at line 365 with BrokenPipe on the QMP socket write. The ruvm process had already gone away when the test sent its next command. The same test then passed 9 times in a row on the same tree, so it looks like a race that only shows when the box is busy and the test runs alongside the rest of the suite. The next step is to keep ruvm's stderr in the test and print it on failure, so we can see why it exited.