vswitchd: Eliminate "can't forward to bad port" when interfaces disappear.

vswitchd: Eliminate "can't forward to bad port" when interfaces disappear.

When an interface was deleted from a datapath by a process other than
vswitchd (which is not supposed to happen), vswitchd would not realize it
and would continue to set up flows for that interface (and leave in place
existing flows).  This caused the kernel to complain "can't forward to bad
port" for each packet on these flows.

Xen triggered this by destroying vifs that were on vswitchd-controlled
datapaths (which removes them from any datapath that they are on).

This fixes the problem, by making vswitchd notice when interfaces
disappear and fixing up the flow table.