CLOUDSTACK-5507: Unable to add XenServer 5.6 host to cloudstack

Fixed "ImportError: No module named cloudstack_pluginlib" on Xenserver 5.6

Signed-off-by: Jayapal <jayapal@apache.org>
Fixed issue in getting interface number in IpAssocVpcCommand

Regression from the below commit

commit 494ccd821d711a2957531d1c33274ed293e4d925
Author: ynojima <mail@ynojima.net>
Date:   Wed Nov 6 11:02:56 2013 -0700
    Bugfix: VR has double NICs connected to Public network