Raise exception if ipv6 prefix is inappropriate for address mode

Address prefix to use with slaac and stateless ipv6 address modes
should be equal to 64 in order to work properly.
The patch adds corresponding validation and fixes unit tests
accordingly.

Change-Id: I6c344b21a69f85f2885a72377171f70309b26775
Closes-Bug: #1357084
(cherry picked from commit 0d8911115e1b722da2f1e92f444e53b22223ee32)
Merge "Raise exception if ipv6 prefix is inappropriate for address mode" into proposed/juno