Don't show configured NICs in the bond dialog

If a NIC is already configured, we probably won't want to
unconfigure it to add it to a bond, much less let users
accidentally select it. Don't show it if it's configured.

Change-Id: I79095cf299f5194b3466d09e35894ecd7affc279
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1078087
Signed-off-by: Ryan Barry <rbarry@redhat.com>