armv7m_systick: delay timer_new to avoid memleaks

There is a memory leak when we call 'device_list_properties' with typename = armv7m_systick. It's easy to reproduce as follow:

virsh qemu-monitor-command vm1 --pretty '{"execute": "device-list-properties", "arguments": {"typename": "armv7m_systick"}}'

This patch delay timer_new to fix this memleaks.

Reported-by: Euler Robot <[e-mail]>
Signed-off-by: Pan Nengyuan <[e-mail]>
Message-id: [e-mail]
Cc: [e-mail]
Reviewed-by: Peter Maydell <[e-mail]>
Signed-off-by: Peter Maydell <[e-mail]>

hw/timer/armv7m_systick.c

···
     memory_region_init_io(&s->iomem, obj, &systick_ops, s, "systick", 0xe0);
     sysbus_init_mmio(sbd, &s->iomem);
     sysbus_init_irq(sbd, &s->irq);
+}
+
+static void systick_realize(DeviceState *dev, Error **errp)
+{
+    SysTickState *s = SYSTICK(dev);
     s->timer = timer_new_ns(QEMU_CLOCK_VIRTUAL, systick_timer_tick, s);
 }
 
···
 
     dc->vmsd = &vmstate_systick;
     dc->reset = systick_reset;
+    dc->realize = systick_realize;
 }
 
 static const TypeInfo armv7m_systick_info = {