Add a 'rate_limiter/group' module that defines the RateLimiterGroup
and a RateLimiterGroupHandle types.
The RateLimiterGroupHandle can be used in place of a RateLimiter to
limit the aggregate bandwidth and/or ops of multiple virtio-blk or
virtio-net queues.
Each RateLimiterGroup has an associated worker thread that broadcasts
an event to each RateLimiterGroupHandle when a RateLimiter is unblocked.
Signed-off-by: Thomas Barrett <tbarrett@crusoeenergy.com>
