kvm tools: Don't dynamically allocate threadpool jobs

kvm tools: Don't dynamically allocate threadpool jobs

To allow efficient use of shorter-term threadpool jobs, don't
allocate them dynamically upon creation. Instead, store them
within 'job' structures.

This will prevent some overhead creating/destroying jobs which live
for a short time.

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