If you want to link a physical network interface to an instance, you need to look into SRIOV or PCI passthrough. See the Networking Guide on docs.openstack.org.

If you are not talking about a physical interface but want some sort of bandwidth guarantee for your instance, Network Quality of Service is the answer. Starting with Newton, you can configure minimum bandwidth on an instance's network port. Again, see the Networking Guide and the spec for this new feature.