qemu-option: qemu_opt_set_bool(): fix code duplication

It will set opt->str in qemu_opt_set_bool, without opt->str, there
will be some potential bugs.

These are uses of opt->str, and what happens when it isn't set:

* qemu_opt_get(): returns NULL, which means "not set".  Bug can bite
  when value isn't the default value.

* qemu_opt_parse(): passes NULL to parse_option_bool(), which treats it
  like "on".  Wrong if the value is actually false.  Bug can bite when
  qemu_opts_validate() runs after qemu_opt_set_bool().

* qemu_opt_del(): passes NULL to g_free(), which is just fine.

* qemu_opt_foreach(): passes NULL to the callback, which is unlikely to
  be prepared for it.

* qemu_opts_print(): prints NULL, which crashes on some systems.

* qemu_opts_to_qdict(): passes NULL to qstring_from_str(), which
  crashes.

It also makes qemu_opt_set_bool more readable by using find_desc_by_name
and opts_accepts_any.

It is based on Luiz's patch and uses Markus's comments. Discussions can
be found at:
http://lists.nongnu.org/archive/html/qemu-devel/2012-07/msg02716.htmlSigned-off-by: NDong Xu Wang <wdongxu@linux.vnet.ibm.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

qemu-option.c

@@ -677,30 +677,21 @@ int qemu_opt_set_bool(QemuOpts *opts, const char *name, bool val)
 {
     QemuOpt *opt;
     const QemuOptDesc *desc = opts->list->desc;
-    int i;
 
-    for (i = 0; desc[i].name != NULL; i++) {
-        if (strcmp(desc[i].name, name) == 0) {
-            break;
-        }
-    }
-    if (desc[i].name == NULL) {
-        if (i == 0) {
-            /* empty list -> allow any */;
-        } else {
-            qerror_report(QERR_INVALID_PARAMETER, name);
-            return -1;
-        }
+    opt = g_malloc0(sizeof(*opt));
+    opt->desc = find_desc_by_name(desc, name);
+    if (!opt->desc && !opts_accepts_any(opts)) {
+        qerror_report(QERR_INVALID_PARAMETER, name);
+        g_free(opt);
+        return -1;
     }
 
-    opt = g_malloc0(sizeof(*opt));
     opt->name = g_strdup(name);
     opt->opts = opts;
-    QTAILQ_INSERT_TAIL(&opts->head, opt, next);
-    if (desc[i].name != NULL) {
-        opt->desc = desc+i;
-    }
     opt->value.boolean = !!val;
+    opt->str = g_strdup(val ? "on" : "off");
+    QTAILQ_INSERT_TAIL(&opts->head, opt, next);
+
     return 0;
 }