performance: 0.867
device: 0.861
graphic: 0.755
mistranslation: 0.723
semantic: 0.621
virtual: 0.618
network: 0.590
debug: 0.590
hypervisor: 0.564
architecture: 0.558
socket: 0.342
boot: 0.305
kernel: 0.295
x86: 0.286
PID: 0.284
user-level: 0.220
i386: 0.168
permissions: 0.163
files: 0.121
peripherals: 0.100
KVM: 0.093
register: 0.086
VMM: 0.075
vnc: 0.063
arm: 0.052
TCG: 0.044
ppc: 0.034
risc-v: 0.031
assembly: 0.025

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