Want ioctl to query packet headroom requirements #843
Description
Currently, we're manually tracking the extra headroom we need Propolis to provide for us at the time it creates Viona interfaces. To keep ourselves in the right (and not end with any regressions here), we should provide an ioctl for use by Propolis, so that it can automatically ask what this value should be. This can be either global, or specified on a per-port basis (if it's sensitive to a given 'NetworkImpl`).