correct migrate_set_speed's args_type

The args_type of migrate_set_speed in qmp-commands.hx is wrong.
When we set migrate speed by json, qemu will be core dumped.

This bug was caused by 07de3e60 and hence affects master only.
Signed-off-by: NWen Congyang <wency@cn.fujitsu.com>
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>

qmp-commands.hx

@@ -495,7 +495,7 @@ EQMP
 
     {
         .name       = "migrate_set_speed",
-        .args_type  = "value:f",
+        .args_type  = "value:o",
         .params     = "value",
         .help       = "set maximum speed (in bytes) for migrations",
         .user_print = monitor_user_noop,