Azure: Fix image names

We need to specify the correct image names -- *.vhdf, not *.vhd -- in
order for them to upload.

15.0 candidate

Reviewed by: lwhsu
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D53684

(cherry picked from commit df84867e8af49437bdc76a0df41d78f5a362cd24)