According to docs, we only support 2 link states for an interface
up/down, 'up' being the default state if link state is unspecified in
domain's XML, so the message when no link state is provided should be
changed a little.
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1141119
(cherry picked from commit d9c7f014ee7c23d25483acf4bce344fae169f1dd)