Preserve device names during instance move

Fix 'move-instance' tool to preserve the instance disk and NIC names
when moving instances from one cluster to another. Also, fix value error
since an instance NIC is an 8-tuple, containing also name and UUID.

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