qemu: make number of CPUs used for qemu configurable

[mvo5]

The qemu backend is currently only using a single CPU and there is no way to configure this. This commit adds support to have a new cpu-count opton in the project that allows customizing this.

The option name is similar to what AWS uses - I'm happy to adjust as needed. Other options might be NumCpu or just Cpu.

This currently only used with qemu but could easily be used in the other backends too if needed.

There is also a slight factory to make it easier testable.