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.h b/hmp.h
index b81439c..a70ac4f 100644
--- a/hmp.h
+++ b/hmp.h
@@ -80,6 +80,7 @@
 void hmp_block_job_resume(Monitor *mon, const QDict *qdict);
 void hmp_block_job_complete(Monitor *mon, const QDict *qdict);
 void hmp_migrate(Monitor *mon, const QDict *qdict);
+void hmp_device_add(Monitor *mon, const QDict *qdict);
 void hmp_device_del(Monitor *mon, const QDict *qdict);
 void hmp_dump_guest_memory(Monitor *mon, const QDict *qdict);
 void hmp_netdev_add(Monitor *mon, const QDict *qdict);