kvm tools: initialize the threadpool job iterator before using

kvm tools: initialize the threadpool job iterator before using

This would fix a bug where the exit function of the threadpool would hang
if no jobs were processed yet and a request to exit was received.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>