Make register and unregister functions return void because
we can check the state of callback object beforehand via
virConnectCloseCallbackDataGetCallback. This can be done
without race conditions if we use higher level locks for registering
and unregistering. The fact they return void simplifies
task of consistent registering/unregistering.
Signed-off-by: NNikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>