qom/object.c: Allow itf cast with num_itfs = 0

num_interfaces only tells you how many interfaces the concrete child class has
(as defined in the TypeInfo). This means if you have a child class which defines
no interfaces of its own, but its parent has interfaces you cannot cast to those
parent interfaces.

Fixed changing the guard to check the class->interfaces list instead (which is
a complete flattened list of implemented interfaces).

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-id: a8c2db3b9b1f3c4bb81aca352b69e33260f36545.1361246206.git.peter.crosthwaite@xilinx.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
