util: add iterators for QemuOpts values

To iterate over all QemuOpts currently requires using a callback
function which is inconvenient for control flow. Add support for
using iterator functions more directly

  QemuOptsIter iter;
  QemuOpt *opt;

  qemu_opts_iter_init(&iter, opts, "repeated-key");
  while ((opt = qemu_opts_iter_next(&iter)) != NULL) {
      ....do something...
  }
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>
Message-id: 20170203120649.15637-8-berrange@redhat.com
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

include/qemu/option.h

@@ -100,6 +100,15 @@ typedef int (*qemu_opt_loopfunc)(void *opaque,
 int qemu_opt_foreach(QemuOpts *opts, qemu_opt_loopfunc func, void *opaque,
                      Error **errp);
 
+typedef struct {
+    QemuOpts *opts;
+    QemuOpt *opt;
+    const char *name;
+} QemuOptsIter;
+
+void qemu_opt_iter_init(QemuOptsIter *iter, QemuOpts *opts, const char *name);
+const char *qemu_opt_iter_next(QemuOptsIter *iter);
+
 QemuOpts *qemu_opts_find(QemuOptsList *list, const char *id);
 QemuOpts *qemu_opts_create(QemuOptsList *list, const char *id,
                            int fail_if_exists, Error **errp);

util/qemu-option.c

@@ -332,6 +332,25 @@ const char *qemu_opt_get(QemuOpts *opts, const char *name)
     return opt ? opt->str : NULL;
 }
 
+void qemu_opt_iter_init(QemuOptsIter *iter, QemuOpts *opts, const char *name)
+{
+    iter->opts = opts;
+    iter->opt = QTAILQ_FIRST(&opts->head);
+    iter->name = name;
+}
+
+const char *qemu_opt_iter_next(QemuOptsIter *iter)
+{
+    QemuOpt *ret = iter->opt;
+    if (iter->name) {
+        while (ret && !g_str_equal(iter->name, ret->name)) {
+            ret = QTAILQ_NEXT(ret, next);
+        }
+    }
+    iter->opt = ret ? QTAILQ_NEXT(ret, next) : NULL;
+    return ret ? ret->str : NULL;
+}
+
 /* Get a known option (or its default) and remove it from the list
  * all in one action. Return a malloced string of the option value.
  * Result must be freed by caller with g_free().