Add ports option

[andig]

Fix #78

[damdo]

How about actually calling the option flag netdev since that's what it actually is?
Otherwise if we really want to call it ports I think we should restrict and sanitize it to only actually be port mappings (ala docker).

After all the gok vm command wraps a qemu vm, so I'm up for exposing some of qemu functionality if required.

[damdo]

But let's see what @stapelberg thinks.

[stapelberg]

How about actually calling the option flag netdev since that's what it actually is?

Yeah, I think that’s a good suggestion (as long as the example in the flag help output is good).

That way, users can use the full power of the netdev qemu option.