performance: 0.867
device: 0.861
graphic: 0.755
semantic: 0.621
network: 0.590
debug: 0.590
other: 0.476
socket: 0.342
boot: 0.305
PID: 0.284
permissions: 0.163
files: 0.121
KVM: 0.093
vnc: 0.063

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