nbd: Pass return value from nbd_handle_list()

nbd: Pass return value from nbd_handle_list()

While it does not make a difference in practice, nbd_receive_options()
generally returns -errno, so it should do that here as well; and the
easiest way to achieve this is by passing on the value returned by
nbd_handle_list().

Signed-off-by: Max Reitz <[email protected]>
Message-Id: <1424887718[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>