xenserver: Fix missing default route problem.

xenserver: Fix missing default route problem.

Taking down the vlan_slave's network device deletes any routes associated
with it, including the default route, and those routes are not restored
when the network device is brought up again later.

So don't take that device down at all.  There's no reason to do so.

We still want to bring that device up after configuring the pif, just in
case it was down, so this commit only deletes the down_netdev, not the
corresponding up_netdev later on.

Second half of bug #1327.