file_size_info: cast 'size' and 'used' to integer

file_size_info: cast 'size' and 'used' to integer

`qemu-img info --output=json` returns the size and used values as integers in
the JSON format, but the regex match converts them to strings.
As we know they only contain digits, we can simply cast them back to integers
after the regex.

The API requires them to be integers.

Signed-off-by: Mira Limbeck <m.limbeck@proxmox.com>
Reviewed-by: Fabian Ebner <f.ebner@proxmox.com>