Add VHDX support to mkimg(1)

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.

Reviewed by: marcel
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D25184