Add a qom property with the same name 'bootindex',
when we remove it form qdev property, things will
continue to work just fine, and we can use qom features
which are not supported by qdev property.

Because usb-storage rely on scsi-disk which is created
in usb_msg_realize_storage(), so we should store the SCSIDevice
pointer in MSDState struct. Only in this way, we can change
the global boot_order_list when we want to change the bootindex
during vm rebooting by calling object_property_set_int(Object(SCSIDevice),).
Signed-off-by: NGonglei <arei.gonglei@huawei.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>