is_netdev is only used as a bool, so make it one.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1468468228-27827-14-git-send-email-eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>