xenserver: Fix use of VLAN as primary management interface.

xenserver: Fix use of VLAN as primary management interface.

xapi insists that the device that carries the IP address for a VLAN is
the bridge that it thinks the VLAN is on.  This commit makes it so.

xapi also will do the ioctl equivalent of "brctl addbr xapi#" if that
bridge appears to be missing when it reconfigures the interface.  If we
return EEXIST or any other error, then it gives up at that point.  So we
have to return "success" to that attempt regardless of whether we really
want to do so.