vswitch: Don't bring up interfaces when adding them to datapaths.

vswitch: Don't bring up interfaces when adding them to datapaths.

Xen expects that the bridge doesn't bring up interfaces when they get
added to a datapath, so bringing them up in our emulation was causing
problems on XenServer.  Keith in particular reports that this commit fixed
boot on XenServer when the management interface was configured to get an
IP address via DHCP.

This change might cause trouble or surprise for other users who do expect
interfaces to be brought up when added to a vswitch, but Keith didn't see
any new problems on XenServer.

Bug #1259.