Fixes Bug #1073547
Nova scheduler's ImagePropertiesFilter based filtering requires the
"supported_instances" stat returned by the driver's get_host_stats method.
This fix implements the get_host_stats method, providing also some
refactoring in the process by adding a HostOps class.
Change-Id: Ia6e73eb3e8ff0be028854fe4fd6d8e305fa1d504