Use new util function for mac_prefix validation

Use new NormalizeAndValidateThreeOctetMacPrefix() util function in
LUNetworkAdd/LUNetworkSetParams to validate network's MAC prefix.
Additionally, move the check in CheckArguments() in the case of
LUNetworkAdd.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Reviewed-by: Michael Hanselmann <hansmi@google.com>