netdev: Don't assume all netdevs are available at runtime.

netdev: Don't assume all netdevs are available at runtime.

Currently we print a warning if a user tries to configure a
netdev that is not in the list that userspace knows about.
However, it is possible that a given netdev maybe be enabled but
when it tries to create a device it finds out that it can't
(not supported by kernel module, hardware not present, etc.).
This makes the behavior the same in both cases.

Signed-off-by: Jesse Gross <jesse@nicira.com>