Add nicparams to the NIC object

Also populate nicparams with a sensible default for imported instances.
Unfortunately we don't know if the bridge set up in old nics is the
default one or not, and we don't have access to the config cluster
object here to verify, so we'll just copy it without questions.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>