CA-116645: We were assuming xenstore contains UTF-8 strings. It doesn't.

So lets sanitise them. This commit pulls in a dependency on uutf by
Daniel Bünzli.

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>
Merge pull request #32 from jonludlam/CA-116645

CA-116645: We were assuming xenstore contains UTF-8 strings. It doesn't.