hw/ppc: clear pending_events on machine reset

The sPAPR machine isn't clearing up the pending events QTAILQ on
machine reboot. This allows for unprocessed hotplug/epow events
to persist in the queue after reset and, when reasserting the IRQs in
check_exception later on, these will be being processed by the OS.

This patch implements a new function called 'spapr_clear_pending_events'
that clears up the pending_events QTAILQ. This helper is then called
inside ppc_spapr_reset to clear up the events queue, preventing
old/deprecated events from persisting after a reset.

Signed-off-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
