How do I convert an .img file to vhd?

VhdTool should be able to do the conversion.

vhdtool.exe /convert myimage.raw

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**
raw                     raw
qcow2                   qcow2
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

Note

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

As indicated above by apple16 there is a quite similar question here that circumvents the problem that VhdTool seems not available anymore. Virtualbox comes with a tool called "VBoxManage" which works just perfect. Provided you've got a plain image file of your disk using e.g. dd, you can do this:

VBoxManage convertfromraw myfile.dd myfile.vhd --format VHD