Fix for bug 738.

Bug fix for port mapping.

This patch fixes a bug that an unexpected port may be mapped if the switch
port to be mapped is specified by its name.

Change-Id: If1643560292e8d57df11b42e24adb1616f00407c
Signed-off-by: Shigeru Yasuda <s-yasuda@da.jp.nec.com>
(cherry picked from commit accd1b97ca6aad5773cfc561e720d1e2651b12cc)