Incorrect worker count in qemu using kvm

When I test emper with a custom kernel in qemu Runtime::getDefaultWorkerCount() returns the number of CPU available to the host not the guest running in qemu.

But running nproc(1) in the guest reports the correct value. That lead me to the corutils nproc source code they use min(configured-cpus, online-cpus, cpus-available-to-process).

We should use something similar.