tests: support QTEST_TRACE env variable

When debugging failing qtests it is useful to be able to turn on trace
output to stderr. The QTEST_TRACE env variable contents get injected
as a '-trace <str>' command line arg

Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220310171821.3724080-4-berrange@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

tests/qtest/libqtest.c

@@ -259,6 +259,9 @@ QTestState *qtest_init_without_qmp_handshake(const char *extra_args)
     gchar *qmp_socket_path;
     gchar *command;
     const char *qemu_binary = qtest_qemu_binary();
+    const char *trace = g_getenv("QTEST_TRACE");
+    g_autofree char *tracearg = trace ?
+        g_strdup_printf("-trace %s ", trace) : g_strdup("");
 
     s = g_new(QTestState, 1);
 
@@ -281,14 +284,15 @@ QTestState *qtest_init_without_qmp_handshake(const char *extra_args)
 
     qtest_add_abrt_handler(kill_qemu_hook_func, s);
 
-    command = g_strdup_printf("exec %s "
+    command = g_strdup_printf("exec %s %s"
                               "-qtest unix:%s "
                               "-qtest-log %s "
                               "-chardev socket,path=%s,id=char0 "
                               "-mon chardev=char0,mode=control "
                               "-display none "
                               "%s"
-                              " -accel qtest", qemu_binary, socket_path,
+                              " -accel qtest",
+                              qemu_binary, tracearg, socket_path,
                               getenv("QTEST_LOG") ? "/dev/fd/2" : "/dev/null",
                               qmp_socket_path,
                               extra_args ?: "");