release: Use standard Azure disk image extension

Azure images are Microsoft VHD files which use the .vhd extension.
We use the mkimg format arg "vhdf" to specify the fixed vhd format, but
the extension should remain .vhd.

Reviewed by: lwhsu, whu
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38761