qemu-img allocates full size on exFAT when metadata preallocation is requested
Description of problem:
`qemu-img` seems to preallocate the full size of a qcow2 image on exFAT rather than just the metadata when that is requested. This was initially seen via libvirt/libvirt#649. exFAT does not support sparse files.
Steps to reproduce:
1. Run command