Avoid duplicate entries in the IP address table

Duplicates are useless and actually cause problems overflowing xenstore's
quota. Also, we should not bother looking at the MIB address table unless
the frontend is CONNECTED or ENABLED since our NetLuid is not necessarily
valid unless we are in one of those states.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Merge pull request #30 from pauldu/master

Avoid duplicate entries in the IP address table