hmp: fix MemdevList memory leak

the memdev_list in hmp_info_memdev() is never freed.
so we use existent method qapi_free_MemdevList() to free it.
and also we can use qapi_free_MemdevList() to replace list loops
to clean up the memdev list in error path.

Signed-off-by: Chen Fan <chen.fan.fnst@cn.fujitsu.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: Hu Tao <hutao@cn.fujitsu.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
diff --git a/hmp.c b/hmp.c
index ba40c75..40a90da 100644
--- a/hmp.c
+++ b/hmp.c
@@ -1715,4 +1715,6 @@
     }
 
     monitor_printf(mon, "\n");
+
+    qapi_free_MemdevList(memdev_list);
 }