I'm not sure for how long this has been the case, but since this is the #1 search result for this question, I'll answer it currently. VHD is currently supported by qemu-img. The argument for VHD is vpc. This was found here http://docs.openstack.org/image-guide/content/ch_converting.html
In case link breaks, here's a copy/past
Converting images from one format to another is generally straightforward.
qemu-img convert: raw, qcow2, VDI, VMDK
The qemu-img convert command can do conversion between multiple formats, including raw, qcow2, VDI (VirtualBox), VMDK (VMWare) and VHD (Hyper-V).
Table 7.1. qemu-img format strings
**Image format** **Argument to qemu-img**
VDI (VirtualBox) vdi
VMDK (VMWare) vmdk
VHD (Hyper-V) vpc
This example will convert a raw image file named centos63.dsk to a qcow2 image file.
$ qemu-img convert -f raw -O qcow2 centos64.dsk centos64.qcow2
To convert from vmdk to raw, you would do:
$ qemu-img convert -f vmdk -O raw centos64.vmdk centos64.img
The -f format flag is optional. If omitted, qemu-img will try to infer the image format.
VBoxManage: VDI (VirtualBox) to raw
If you've created a VDI image using VirtualBox, you can convert it to raw format using the VBoxManage command-line tool that ships with VirtualBox. On Mac OS X, VirtualBox stores images by default in the ~/VirtualBox VMs/ directory. The following example creates a raw image in the current directory from a VirtualBox VDI image.
$ VBoxManage clonehd ~/VirtualBox\ VMs/fedora18.vdi fedora18.img --format raw