XenAPI: Add another plugin directory

Buildroot on 64-bit systems may use /usr/lib64 to store the XAPI plugins
Add this as an option to search for.

The list is getting of acceptable paths is getting longer but some work is going
on in XAPI to allow us to query for this path, which will mean we can get
rid of this list in future.

Change-Id: I79aafb6a86032c7ab04937c9e9bec08661ecdefa
Merge "XenAPI: Add another plugin directory"