This command creates/deletes/modifies bandwidth groups of the given virtual machine:
VBoxManage bandwidthctl <uuid|vmname> --name <name> [--add disk [--delete] [--limit MB/s]
See Section 5.8, “Limiting bandwidth for disk images” for an introduction to bandwidth limits. The parameters mean:
The VM UUID or VM Name. Mandatory.
Name of the bandwidth group. Mandatory.
Creates a new bandwdith group with the given type.
Deletes a bandwdith group if it isn't used anymore.
Sets the limit for the given group to the specified amount. Can be changed while the VM is running.