Add support for the suspend event

Add support for the suspend event

This patch introduces a new event type for the QMP event
SUSPEND:

    VIR_DOMAIN_EVENT_ID_PMSUSPEND

The event doesn't take any data, but considering there might
be reason for wakeup in future, the callback definition is:

typedef void
(*virConnectDomainEventSuspendCallback)(virConnectPtr conn,
                                        virDomainPtr dom,
                                        int reason,
                                        void *opaque);

"reason" is unused currently, always passes "0".