Add check for duplicate MACs in instance add

Currently LUAddInstance doesn't check for duplicate MACs, and it fails
during the Exec() phase when trying to add the instance to the config
(ConfigWriter checks for this). This patch copies the code from
LUModifyInstance (which already does it).

This fixed issue #70.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Olivier Tharan <olive@google.com>