qmp: wait for POWERDOWN event in ExecuteSystemPowerdown()

ExecuteSystemPowerdown issues `system_powerdown` and waits
for `SHUTDOWN`. The event emitted is `POWERDOWN` per spec.

Without this we get an error even though the VM has shutdown
gracefully.

Per QEMU spec:

```

POWERDOWN (Event)

Emitted when the virtual machine is powered down through the power
control system, such as via ACPI.

Since

0.12

Example

<- { "event": "POWERDOWN",
     "timestamp": { "seconds": 1267040730, "microseconds": 682951 } }

SHUTDOWN (Event)

Emitted when the virtual machine has shut down, indicating that qemu is
about to exit.

Arguments

guest: boolean
If true, the shutdown was triggered by a guest request (such as a
guest-initiated ACPI shutdown request or other hardware-specific action)
rather than a host request (such as sending qemu a SIGINT). (since 2.10)
reason: ShutdownCause
The ShutdownCause which resulted in the SHUTDOWN. (since 4.0)
Note

If the command-line option “-no-shutdown” has been specified, qemu will
not exit, and a STOP event will eventually follow the SHUTDOWN event

Since

0.12

Example

<- { "event": "SHUTDOWN", "data": { "guest": true },
     "timestamp": { "seconds": 1267040730, "microseconds": 682951 } }

```

Signed-off-by: Manohar Castelino <mcastelino@apple.com>

qemu/qmp.go

@@ -761,7 +761,7 @@ func (q *QMP) ExecuteCont(ctx context.Context) error {
 // This function will block until the SHUTDOWN event is received.
 func (q *QMP) ExecuteSystemPowerdown(ctx context.Context) error {
 	filter := &qmpEventFilter{
-		eventName: "SHUTDOWN",
+		eventName: "POWERDOWN",
 	}
 	return q.executeCommand(ctx, "system_powerdown", nil, filter)
 }

qemu/qmp_test.go

@@ -802,7 +802,7 @@ func TestQMPSystemPowerdown(t *testing.T) {
 	disconnectedCh := make(chan struct{})
 	buf := newQMPTestCommandBuffer(t)
 	buf.AddCommand("system_powerdown", nil, "return", nil)
-	buf.AddEvent("SHUTDOWN", time.Millisecond*100,
+	buf.AddEvent("POWERDOWN", time.Millisecond*100,
 		nil,
 		map[string]interface{}{
 			"seconds":      seconds,