devstack: Add paths specific to XenServer Core

 * XenServer Core keeps its Xapi plugins in a different path to XenServer,
   in order to comply with the Filesystem Hierarchy Standard.

 * XenServer Core mounts devices under /dev/xen/blktap-2 in dom0,
   whereas XenServer mounts them under /dev/sm.

Change-Id: I0a66cd03f000f19c8ff63a8ae4231c00fda88380
Merge "devstack: Add paths specific to XenServer Core"