vswitchd: Mirror nothing, not everything, if mirror ports don't exist.

vswitchd: Mirror nothing, not everything, if mirror ports don't exist.

If all of the ports specified as mirror selection criteria actually do not
exist, then until now the bridge would mirror all incoming packets (on
specified VLAN(s), if any).  This matches the behavior that occurs if no
mirror selection ports were specified at all, and so it makes a certain
amount of logical sense.

But it is far more likely that the user simply misspelled a port name, or
specified the name of a port that does not always exist.  In fact we have
seen this behavior in practice when the controller has not caught up to
the switch's current configuration.  So this commit changes the bridge to
instead disable a mirror if ports are specified and none of those ports
exist.

Bug #1904.