Extend the CheckArguments phase of LUInstanceCreate and CheckPrereq
phase of LUInstanceSetParams to also check if the name parameters of
disks and NICs are unique and valid.

Signed-off-by: Christos Stavrakakis <cstavr@grnet.gr>
Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Reviewed-by: Helga Velroyen <helgav@google.com>