change file_size_info sub to use qemu-img info json decoding

change file_size_info sub to use qemu-img info json decoding

Using the json output, as suggested by Thomas, we now die if the decoding
fails and, if not, all return values are set to the corresponding decoded
values. That should prevent any unforeseen null size values, except if
qemu-img info reports it, which we then consider as valid.

Signed-off-by: Tim Marx <t.marx@proxmox.com>