vswitch: Enable bond slaves based on carrier status, not up/down.

vswitch: Enable bond slaves based on carrier status, not up/down.

Whether a bond slave is enabled should be based on whether the device's
PHY sees carrier, not based on whether the device is configured up or down.
(Note that a device that is configured down will always see "no carrier").
Otherwise a device that is up but has no carrier will initially be enabled,
which does not make sense.

This has no effect on interfaces that are not bond slaves, because the
"enabled" setting is used only by bond slaves.

Bug #1247.