Update XenBus to #9 542a9bab68f335e8134623d9cd964cd4b29896a7

Signed-off-by: Ben Chalmers <Ben.Chalmers@citrix.com>
Merge pull request #15 from benchalmers/xenbus9

Update XenBus to #9 542a9bab68f335e8134623d9cd964cd4b29896a7

Approved-By: Ben Chalmers <Ben.Chalmers@citrix.com>