vmm: use RateLimiterGroup for virtio-blk devices

Add a 'rate_limit_groups' field to VmConfig that defines a set of
named RateLimiterGroups.

When the 'rate_limit_group' field of DiskConfig is defined, all
virtio-blk queues will be rate-limited by a shared RateLimiterGroup.
The lifecycle of all RateLimiterGroups is tied to the Vm.
A RateLimiterGroup may exist even if no Disks are configured to use
the RateLimiterGroup. Disks may be hot-added or hot-removed from the
RateLimiterGroup.

When the 'rate_limiter' field of DiskConfig is defined, we construct
an anonymous RateLimiterGroup whose lifecycle is tied to the Disk.
This is primarily done for api backwards compatability. Importantly,
the behavior is not the same! This implementation rate_limits the
aggregate bandwidth / iops of an individual disk rather than the
bandwidth / iops of an individual queue of a disk.

When neither the 'rate_limit_group' or the 'rate_limiter' fields of
DiskConfig is defined, the Disk is not rate-limited.

Signed-off-by: Thomas Barrett <tbarrett@crusoeenergy.com>

docs/io_throttling.md

@@ -42,3 +42,14 @@ actual rate limit users get can be as low as
 generally advisable to keep `bw/ops_refill_time` larger than `100 ms`
 (`cool_down_time`) to make sure the actual rate limit is close to users'
 expectation ("refill-rate").
+
+## Rate Limit Groups
+It is possible to throttle the aggregate bandwidth or operations
+of multiple virtio-blk devices using a `rate_limit_group`. virtio-blk devices may be
+dynamically added and removed from a `rate_limit_group`. The following example
+demonstrates how to throttle the aggregate bandwidth of two disks to 10 MiB/s.
+```
+--disk path=disk0.raw,rate_limit_group=group0 \
+       path=disk1.raw,rate_limit_group=group0 \
+--rate-limit-group bw_size=1048576,bw_refill_time,bw_refill_time=100
+```