Introduce RateLimiter to virtiofsd
RateLimiter is a good tool to do ops and bandwidth limit. It's orignally from Firecracker. RateLimiter uses a common way to achieve its goal: token bucket. More details see: https://codecatalog.org/articles/firecracker-rate-limiting/
virtiofsd is widely used in micro-vm field, but it currently doesn't have a way to do QoS, especially in buffered IO case. This patchset introduces RateLimiter to it so that we can control the ops and bandwidth accurately. Note, currently it is only applied in thread_pool_size=0 case.