Hello,
There are various formats of virtual images, .ISO it is simply the most popular, almost a standard.
The other day I came across an image in format .MDF, and I needed to convert it to . ISO.
Here are the steps:
1. Install see below
2. Suppose the .MDF It's in "/home/user/image.mdf". We put the following:
- iat /home/user/image.mdf /home/user/image.iso
And that's it, it will become ours 
regards