vl: add tcg_enabled() for tcg related code

Need to disable the tcg related code in the vl.c if the
disable-tcg option is added into ./configure command.

Signed-off-by: Yang Zhong <yang.zhong@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

vl.c

@@ -3933,6 +3933,10 @@ int main(int argc, char **argv, char **envp)
                 configure_rtc(opts);
                 break;
             case QEMU_OPTION_tb_size:
+                if (!tcg_enabled()) {
+                    error_report("TCG is disabled");
+                    exit(1);
+                }
                 if (qemu_strtoul(optarg, NULL, 0, &tcg_tb_size) < 0) {
                     error_report("Invalid argument to -tb-size");
                     exit(1);
@@ -4481,7 +4485,9 @@ int main(int argc, char **argv, char **envp)
         qemu_opts_del(icount_opts);
     }
 
-    qemu_tcg_configure(accel_opts, &error_fatal);
+    if (tcg_enabled()) {
+        qemu_tcg_configure(accel_opts, &error_fatal);
+    }
 
     if (default_net) {
         QemuOptsList *net = qemu_find_opts("net");