is off, this is just the control net interface (xenbr0). But when
XENVIFROUTING is on, we want to listen on the control net bridge plus
all of the container vifs. Since these are not created until the
container is started, we have to call restartDHCP from emulab-cnet (we
were already doing that), and now we also call reconfigDHCP() when the
contain is destroyed so that interface list is correct (note that DHCPD
does not seem to care if an interface dissappears, or even if an
interface does not exist when starting.

The main point here, is that on shared nodes we have to restrict the
number of interfaces that DHCPd listens on (or even looks at) since it
can be 100s, and dhcpd was taking well over a minute to start up each
time.

Aside; minor change to not look at the IP config for bridges, just the
mac. Takes to long when there are 100s of bridges.