Convert instance_list rpc to new style result

Since backend.GetInstanceList() is used both as RPC endpoint and as
internal function, it can't return (status, value). Instead it returns
only valid instance info, and failures are denoted by exceptions; and
the ganeti-noded function adds the (True,) status.

The patch also fixes a typo.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>