devstack plugin: properly restore xtrace
devstack plugin: properly restore xtrace

Change-Id: Id4a63433e9c767e669eee57d82e86cc71b23316a

stackforge/networking-bagpipe-l2,openstack/networking-bagpipe,openstack/networking-bagpipe-l2,stackforge/networking-bagpipe-l2,openstack/networking-bagpipe,openstack/networking-bagpipe-l2