Extend creation UI with additional VHD support

[ananace]

A 2 minute mockup;

Options should include;

• New VHD, which acts like the current UI
• Source VHD, copies said source to act as the base of the VM disk
• Existing VHD, adds an already existing VHD as a disk on the new VM

This should make a workflow like the one discussed in #12 possible.