Add support for event tray moved of removable disks

Add support for event tray moved of removable disks

This patch introduces a new event type for the QMP event
DEVICE_TRAY_MOVED, which occurs when the tray of a removable
disk is moved (i.e opened or closed):

    VIR_DOMAIN_EVENT_ID_TRAY_CHANGE

The event's data includes the device alias and the reason
for tray status' changing, which indicates why the tray
status was changed. Thus the callback definition for the event
is:

enum {
    VIR_DOMAIN_EVENT_TRAY_CHANGE_OPEN = 0,
    VIR_DOMAIN_EVENT_TRAY_CHANGE_CLOSE,

\#ifdef VIR_ENUM_SENTINELS
    VIR_DOMAIN_EVENT_TRAY_CHANGE_LAST
\#endif
} virDomainEventTrayChangeReason;

typedef void
(*virConnectDomainEventTrayChangeCallback)(virConnectPtr conn,
                                           virDomainPtr dom,
                                           const char *devAlias,
                                           int reason,
                                           void *opaque);