Move string formatting out of LUQueryInstances

Currently, LUQueryInstances will provide strings for its results. This makes it
hard for other consumers than "gnt-instance list" to use the OpQueryInstances
opcode for whatever they wish to.

The change moves the formatting in five of the six cases where this happens to
the actual gnt-instance.ListInstances procedure and removes the unconditional
convert to string in LUQueryInstances.Exec()

Reviewed-by: imsnah