nbd: Always call "close_fn" in nbd_client_new

nbd: Always call "close_fn" in nbd_client_new

Rename the parameter "close" to "close_fn" to disambiguous with
close(2).

This unifies error handling paths of NBDClient allocation:
nbd_client_new will shutdown the socket and call the "close_fn" callback
if negotiation failed, so the caller don't need a different path than
the normal close.

The returned pointer is never used, make it void in preparation for the
next patch.

Signed-off-by: Fam Zheng <[email protected]>
Message-Id: <1452760863[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>