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-commands.hx b/hmp-commands.hx
index 81723c8..d0ce6a5 100644
--- a/hmp-commands.hx
+++ b/hmp-commands.hx
@@ -1037,8 +1037,7 @@
         .args_type  = "netdev:O",
         .params     = "[user|tap|socket],id=str[,prop=value][,...]",
         .help       = "add host network device",
-        .user_print = monitor_user_noop,
-        .mhandler.cmd_new = do_netdev_add,
+        .mhandler.cmd = hmp_netdev_add,
     },
 
 STEXI