Add support for the fixed image type. The fixed image is effectively
a raw image with a VHD footer appended. There's little value that I
can see to use the fixed image type, but in order to make VHD images
for use by Microsoft's Azure platform, they must be fixed VHD images.

Support has been added by refactoring the code to re-use common code
and by adding a second output format structure. To created fixed VHD
images, specify "vhdf" as the output format.