add next free device option to qemu-img

I'd like to propose an option to be added to qemu-img which returns the next free NBD (the device file) very similar to losetup -f. It would make life a lot easier.

Followers of this enhancement request might be interested in the following workaround: http://stackoverflow.com/questions/22535222/next-free-device-option-for-qemu-nbd/