Make the semantics of Activations more like Mirage-Xen

A user of this library would never want to call Activations.run -
Mirage-Xen runs this automatically, and so we do here as well now.

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>
Merge pull request #36 from jonludlam/mirageyactivations

Make the semantics of Activations more like Mirage-Xen