Support read and write VHD_DIFFERENCING

Now qemu only support use VHD_FIXED and VHD_DYNAMIC file, this patch
Add the ability to use VHD_DIFFERENCING file

In VHD_DIFFERENCING file, the parent information is added, so read the
parent pathx

when read VHD_DIFFERENCING file, will judge if the data in child or
parent

when write VHD_DIFFERENCING file, will write child file and bitmap.

Signed-off-by: Ding xiao <ssdxiao@163.com>