virInterfacePtr iface;

iface = virInterfaceLookupByName("eth2");
if (!iface) {
    printf("Interface eth2 not found.\n");
} else {
    if (virInterfaceDestroy(iface) != 0) {
        fprintf(stderr, "failed to destroy (deactivate) interface eth2.\n");
    } else
        /* do whatever you wanted to do with interface down */
        if (virInterfaceCreate(iface) != 0) {
        fprintf(stderr, "failed to create (activate) interface eth2.\n");
        }
    }
    free(iface);
}