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/net.h b/net.h
index 7ee97e9..1eb9280 100644
--- a/net.h
+++ b/net.h
@@ -170,7 +170,8 @@
 void net_cleanup(void);
 void net_host_device_add(Monitor *mon, const QDict *qdict);
 void net_host_device_remove(Monitor *mon, const QDict *qdict);
-int do_netdev_add(Monitor *mon, const QDict *qdict, QObject **ret_data);
+void netdev_add(QemuOpts *opts, Error **errp);
+int qmp_netdev_add(Monitor *mon, const QDict *qdict, QObject **ret);
 int do_netdev_del(Monitor *mon, const QDict *qdict, QObject **ret_data);
 
 #define DEFAULT_NETWORK_SCRIPT "/etc/qemu-ifup"