Remove callbacks from virt driver(s)

There are unused callback parameters in the virt driver (nova.virt.driver:ComputeDriver). Specifically:

    def pause(self, instance, callback):
    def unpause(self, instance, callback):
    def suspend(self, instance, callback):
    def resume(self, instance, callback):
    def rescue(self, context, instance, callback, network_info):
    def unrescue(self, instance, callback, network_info):

I believe the original intention was to pass a callback and then perform a non-blocking call at the virt layer...but none of the drivers support this I would like to remove them to reduce programmer confusion. XenAPI comes the closest, but it does a blocking loop before finally calling the callback...essentially making the callback useless.