virtio-devices: Move the 'rate_limiter' module to its own crate

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>

Cargo.toml

@@ -76,6 +76,7 @@ members = [
     "option_parser",
     "pci",
     "qcow",
+    "rate_limiter",
     "vhost_user_backend",
     "vhost_user_block",
     "vhost_user_net",