Check that device names are unique and valid

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>