vswitch: Keep existing datapaths when starting up.

vswitch: Keep existing datapaths when starting up.

Until now, vswitchd has deleted all existing datapaths when it started up,
and then re-created the ones that are actually configured.  This is a very
"clean", conservative design, but it has undesirable effects in practice.
In particular, if any datapath has a local port with an IP address
configured on it, then deleting the datapath and recreating it will drop
that IP address, which can mean that your machine just lost connectivity
to the outside world.

So, with this commit, now vswitchd only deletes datapaths at startup that
don't have configured bridges, preserving local port IP addresses.