Add support for nics.*.max_bandwidth and nics.*.priority properties to vmadm

[brianewell]

It might be useful to expose the link properties 'maxbw' (as max_bandwidth) and 'priority' so that they can be configured via vmadm.

As it is now, I've been using dladm to configure these properties on zones after the fact, but the state is reset between reboots, making it somewhat impractical for the long term.

@sjorge I'm willing to give this a go, despite having any previous experience inside of vmadm, so this'll be fun.

[sjorge]

Good luck, my tinkering with vmadm were usually filled with chaos and frustration.
You probably meant to tag some of the Joyent people I think @bahamat has done the most tinkering there recently.