[CA-122026] Lite aganet stability improvements

Do not crash when permissions stop you advertising features
Cope with reading fron 0 length xenstore entries
Use XsLog not XsLogMsg

Signed-off-by: Ben Chalmers <Ben.Chalmers@citrix.com>
Merge pull request #4 from benchalmers/ca-122026

[CA-122026] Lite aganet stability improvements