Fix #908: cleanup OVS ports in tap_unplug

Fix #908: cleanup OVS ports in tap_unplug

tap_unplug() is only called in hotplugging functions which
perform a tap_plug() afterwards, and and in qemu-server's
bridge-down script.
The OVS port cleanup for non-firewall ports was done only in
tap_plug(), which means that in the case when qemu exits and
the tap device disappears, the OVS port for it stays.

With this patch the cleanup happens in both tap_unplug() and
tap_plug() to avoid problems with upgrades on systems
where there have already been ports leaked.