bhyve: net_backends, automatically IFF_UP tap devices

If you want communications with the outside world and tell bhyve to
create an interfaces then it should be usable as well.
Rather than relying on the sysctl net.link.tap.up_on_open automatically
try to IFF_UP the opened tap device.

MFC after: 10 days
Reviewed by: markj, grehan
Differential Revision: https://reviews.freebsd.org/D31342