hw/misc/vmcoreinfo: Add comment about reset handler

hw/misc/vmcoreinfo: Add comment about reset handler

The VM coreinfo device does not sit on a bus, so it won't be
reset automatically. This is why it calls qemu_register_reset().

Add a comment about it, so we don't convert its reset handler
to a DeviceReset method.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20191010131527.32513-9-philmd@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>