How to create vhdx differencing disk using qemu-img

[qi zhou]

When I create vhdx differencing disk using qemu-img, It says
 qemu-img: xxx.vhd Backing file not supported for file format 'vhdx'

The command I used is
qemu-img create -f vhdx -b test.vhdx test-snapshot.vhdx

Here is my questions
1. Is vhdx format [full] supported by qemu ?
2. If not, is there any easy way to implement differencing disk of vhdx in 
qemu-img ?
3. Is there any other tools support vhdx on linux ?