Azure: Fix image names

[cperciva]

Summary

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

15.0 candidate

MFC after: 3 days

[lwhsu]

I am ok with this change, but please note that in Azure, it requires the "source image" file extension be .vhd so even the local file is .vhdf, we still need it to be uploaded as .vhd as set line 49.

[cperciva]

In D53684#1225631, @lwhsu wrote:

I am ok with this change, but please note that in Azure, it requires the "source image" file extension be .vhd so even the local file is .vhdf, we still need it to be uploaded as .vhd as set line 49.

Yes, I was careful to not change the --name we use to upload it. The .vdhf is necessary because that's the file name created by the code in Makefile.vm.