rate_limiter: add RateLimiterGroup

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>

rate_limiter/src/lib.rs

@@ -53,6 +53,9 @@ use std::sync::Mutex;
 use std::time::{Duration, Instant};
 use vmm_sys_util::timerfd::TimerFd;
 
+/// Module for group rate limiting.
+pub mod group;
+
 #[derive(Debug)]
 /// Describes the errors that may occur while handling rate limiter events.
 pub enum Error {