monitor: Use traditional command interface for HMP device_add

All QMP commands use the "new" handler interface (mhandler.cmd_new).
Most HMP commands still use the traditional interface (mhandler.cmd),
but a few use the "new" one.  Complicates handle_user_command() for no
gain, so I'm converting these to the traditional interface.

For device_add, that's easy: just wrap the obvious hmp_device_add()
around do_device_add().

monitor_user_noop() is now unused, drop it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
diff --git a/hmp-commands.hx b/hmp-commands.hx
index 5257969..eff2f97 100644
--- a/hmp-commands.hx
+++ b/hmp-commands.hx
@@ -653,8 +653,7 @@
         .args_type  = "device:O",
         .params     = "driver[,prop=value][,...]",
         .help       = "add device, like -device on the command line",
-        .user_print = monitor_user_noop,
-        .mhandler.cmd_new = do_device_add,
+        .mhandler.cmd = hmp_device_add,
         .command_completion = device_add_completion,
     },