Add autodetection for vhd files.

[redstar]

vhd files are often used as storage containers. This PR adds a new commandline switch to check for vhd files. This has the following advantages:

• Dynamic vhd files are rejected so you can't destroy them.
• Size of the image is adjusted to exclude the management block.

[ecashin]

Hi. Have you had a chance to consider my comments? Thanks much.

[ecashin]

I guess I will close this branch soon, unless @redstar becomes active.