correct the arguments of migrate_speed

When we set migrate_speed by json, we receive the following
error message:
libvirtError: internal error unable to execute QEMU command
'migrate_set_speed': Invalid parameter type, expected: number

The reason is that: the arguments of migrate_set_speed
by json is json number, not json string.
Signed-off-by: NWen Congyang <wency@cn.fujitsu.com>

AUTHORS

@@ -135,6 +135,7 @@ Patches have also been contributed by:
   John Morrissey       <jwm@horde.net>
   Osier Yang           <jyang@redhat.com>
   Kamezawa Hiroyuki    <kamezawa.hiroyu@jp.fujitsu.com>
+  Wen Congyang         <wency@cn.fujitsu.com>
 
   [....send patches to get your name here....]
 

src/qemu/qemu_monitor_json.c

@@ -1453,17 +1453,11 @@ int qemuMonitorJSONSetMigrationSpeed(qemuMonitorPtr mon,
                                      unsigned long bandwidth)
 {
     int ret;
-    char *bandwidthstr;
     virJSONValuePtr cmd;
     virJSONValuePtr reply = NULL;
-    if (virAsprintf(&bandwidthstr, "%lum", bandwidth) < 0) {
-        virReportOOMError();
-        return -1;
-    }
     cmd = qemuMonitorJSONMakeCommand("migrate_set_speed",
-                                     "s:value", bandwidthstr,
+                                     "U:value", bandwidth * 1024ULL * 1024ULL,
                                      NULL);
-    VIR_FREE(bandwidthstr);
     if (!cmd)
         return -1;