libxc: add ability to query OS interface for "fakeness"

libxc: add ability to query OS interface for "fakeness"

i.e. not running on a real hypervisor

Allows users of the library to adjust behaviour. I don't especially
like this violation of the abstraction but both oxenstored and xapi
use this to avoid difficult to simulate operations when running on the
simulator.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Ian Jackson <ian.jackson.citrix.com>