flow_caches_stress_test: Fix undefined name 'prompt'

The commit 2687b22a "qemu.tests:modify netperf related script"
assigns shell prompt to variable 'shell_prompt',
but pass 'prompt' to utils_netperf.NetperfServer().
Signed-off-by: NWei Jiangang <weijg.fnst@cn.fujitsu.com>

qemu/tests/flow_caches_stress_test.py

@@ -83,7 +83,7 @@ def run(test, params, env):
     passwd = params.get("hostpasswd", "123456")
     client = params.get("shell_client", "ssh")
     port = params.get("shell_port", "22")
-    shell_prompt = params.get("shell_prompt", "^root@.*[\#\$]\s*$|#")
+    prompt = params.get("shell_prompt", "^root@.*[\#\$]\s*$|#")
     linesep = params.get("shell_linesep", "\n").decode('string_escape')
     status_test_command = params.get("status_test_command", "echo $?")