vswitchd: Skip updelay on slave when only a single bond slave is up.

vswitchd: Skip updelay on slave when only a single bond slave is up.

If a network device takes a few seconds to detect carrier, as some do, then
when bringing up a network device and then immediately adding that device
to a bridge, the bond code would start out with that slave considered down
and apply the full updelay to it before bringing it up.  With the 31-second
updelay set by XenServer, this is excessive: we end up having no
connectivity at all for 31 seconds even though there is no reason for it.

This commit makes the bond code disregard the updelay when an interface
comes up on a bond that has no enabled interfaces, and updates the
documentation to match.

Part of bug #1566.