To support I/O throttling on virt-net devices, we need to use the
'rate_limiter' module from the 'net_utils' crate. Given the
'virtio-devices' crate has dependency on the 'net_utils', we will need
to move the 'rate_limiter' module out of the 'virtio-devices' crate to
avoid circular dependency issue. Considering the 'rate_limiter' is not
virtio specific and could be reused for non virtio devices, we move it
to its own crate.
Signed-off-by: Bo Chen <chen.bo@intel.com>
