xenserver: Bring up bond slave device before adding to bond.

xenserver: Bring up bond slave device before adding to bond.

Until now, we've added network devices to the ovs-vswitchd configuration
file before bringing them up.  This works suboptimally for bond slaves,
because vswitchd takes the initial carrier state of the bond slaves as
whether to initially enable or disable them, and a device that is down
always reports "no carrier".  So this commit changes interface-reconfigure
to bring up bond slaves before adding them to the configuration file,
which gives them a chance to detect carrier before vswitchd checks for it.

This might still be an imperfect situation, because it takes quite a while
(maybe 1 second?) for some network devices to detect carrier after being
brought up.  But the kernel implementation of bonding seems to do the very
same thing, so we should be at least no worse than it.

Incidentally, this is only important for bonds that have long updelays.
XenServer uses an updelay of 31 seconds (!), so it is especially important
there.

Bug #1247.