vl.c: fix '-cpu ?' segfault

Fix stupid copy&paste mistake at commit
ecf40bea: I moved code around but kept
"optarg" on the cpu_list() call.
Reported-by: NJiri Denemark <jdenemar@redhat.com>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

vl.c

@@ -3196,7 +3196,7 @@ int main(int argc, char **argv, char **envp)
     cpudef_init();
 
     if (cpu_model && *cpu_model == '?') {
-        list_cpus(stdout, &fprintf, optarg);
+        list_cpus(stdout, &fprintf, cpu_model);
         exit(0);
     }