Use wfi to sleep a cpu if no process is found while cycling pool

[Virtual-Machine]

This pull request is to propose a minor change on how the scheduler works.

This PR should resolve #8 in which CPU utilization is high when running in QEMU.

This is a fix proposed in xv6-riscv as well (see https://github.com/mit-pdos/xv6-riscv/issues/15) and is at least personally verified as lowering CPU utilization on my Macbook. I am not sure if there are any possible unintended side effects of this change, so additional testing and verification may be warranted.

[sword-jin]

Awesome man, This fix is gona save my PC.