QemuImgInfo is realized by parsing the output string of 'qemu-img info
**.img' line by line, it's complicated and improvment can be done. The
command 'qemu-img info **.img --output json' will produce json style
output and arguments can be parsed easily by calling json.loads. This
patch add support json format output parsing, to simplify the code and
make it more clear and readable.
Change-Id: Iefa139dc4bcea864cb86371d8a24df0e4695169c
Closes-Bug:#1590252