Add VHDX support to mkimg(1)

[gonzo]

Summary

VHDX is the successor of Microsoft's VHD file format. It increases
maximum capacity of the virtual drive to 64TB and introduces features
to better handle power/system failures.

VHDX is the required format for 2nd generation Hyper-V VMs.

Test Plan

Generated VHDX image based on the release image and successfully booted
2nd generation Hyper-V VM using it as a virtual drive.

[marcel]

Very nice!