bhyve: net_backends, automatically IFF_UP tap devices

[bz]

Summary

Rather than relying on the sysctl net.link.tap.up_on_open automatically
try to IFF_UP the opened tap device.
When moving installations a lot in devleopment environments forgetting
to set the sysctl is annoying and having to in first place too.
If you want communications with the outside world and tell bhyve to
create an interfaces then it should be usable as well.

MFC after: 10 days

Test Plan

It worked when I tried.

[markj]

usr.sbin/bhyve/net_backends.c

229

Why static?

279

Ditto below.

[bz]

Address @markj 's comments.

[bz]

usr.sbin/bhyve/net_backends.c

229

Because I was tired at 2AM? Sorry.

[markj]

Thanks, this seems reasonable to me.

[bz]

Thanks @markj . Are there any POLA concerns here @grehan or can this go in?

[grehan]

No POLA concerns: this is fine. Thanks bz.