vswitch: interface-reconfigure: bring down physical interfaces

vswitch: interface-reconfigure: bring down physical interfaces

This should be done when bringing down the last PIF which uses a
datapath in order to account for VLAN PIFs sharing a datapath.

The logic in bring_down() already acounts for this requirement by
clearing the dp variable if the datapath is still required so if we
get as far as deconfiguring that datapath then it is also correct to
bring down the physical devices.

With unit test suite update by Ben Pfaff.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>