XenAPI:  Find location for XenAPI plugins

Plugins can be installed in different places in dom0; re-enable the
use of xapi_plugin_location

Change-Id: If94ec6b250ad5b4f7730cfbf443d937eb85feb5c
Merge commit 'refs/changes/49/81849/1' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch