CP-4480: Add a call to check whether PV drivers have started

This is modelled after a similarly named call in libxl. We
check the CALLBACK_IRQ status for HVM domains. This requires
no changes to libxc.

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>
Merge pull request #131 from jonludlam/CP-4480

CP-4480: Add a call to check whether PV drivers have started