spapr.c: send QAPI event when memory hotunplug fails

spapr.c: send QAPI event when memory hotunplug fails

Recent changes allowed the pSeries machine to rollback the hotunplug
process for the DIMM when the guest kernel signals, via a
reconfiguration of the DR connector, that it's not going to release the
LMBs.

Let's also warn QAPI listerners about it. One place to do it would be
right after the unplug state is cleaned up,
spapr_clear_pending_dimm_unplug_state(). This would mean that the
function is now doing more than cleaning up the pending dimm state
though.

This patch does the following changes in spapr.c:

- send a QAPI event to inform that we experienced a failure in the
  hotunplug of the DIMM;

- rename spapr_clear_pending_dimm_unplug_state() to
  spapr_memory_unplug_rollback(). This is a better fit for what the
  function is now doing, and it makes callers care more about what the
  function goal is and less about spapr.c internals such as clearing
  the pending dimm unplug state.

Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20210302141019.153729-3-danielhb413@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>