qapi: convert netdev_add

This is not a full QAPI conversion, but an intermediate step.

In essence, do_netdev_add() is split into three functions:

 1. netdev_add(): performs the actual work. This function is fully
    converted to Error (thus, it's "qapi-friendly")

 2. qmp_netdev_add(): the QMP front-end for netdev_add(). This is
    coded by hand and not auto-generated (gen=no in the schema). The
    reason for this it's a lot easier and simpler to with QemuOpts
    this way

 3. hmp_netdev_add(): HMP front-end.

This design was suggested by Paolo Bonzini.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-By: Laszlo Ersek <lersek@redhat.com>
diff --git a/hmp.c b/hmp.c
index 42ced2a..7a4e25f 100644
--- a/hmp.c
+++ b/hmp.c
@@ -14,6 +14,8 @@
  */
 
 #include "hmp.h"
+#include "net.h"
+#include "qemu-option.h"
 #include "qemu-timer.h"
 #include "qmp-commands.h"
 
@@ -969,3 +971,22 @@
                           &errp);
     hmp_handle_error(mon, &errp);
 }
+
+void hmp_netdev_add(Monitor *mon, const QDict *qdict)
+{
+    Error *err = NULL;
+    QemuOpts *opts;
+
+    opts = qemu_opts_from_qdict(qemu_find_opts("netdev"), qdict, &err);
+    if (error_is_set(&err)) {
+        goto out;
+    }
+
+    netdev_add(opts, &err);
+    if (error_is_set(&err)) {
+        qemu_opts_del(opts);
+    }
+
+out:
+    hmp_handle_error(mon, &err);
+}