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

release/Makefile.azure

@@ -39,7 +39,7 @@ azure-check-depends:
 .endif
 
 .for _FS in ${AZURE_FSLIST}
-AZURE_IMG_${_FS}=	${.OBJDIR}/azure.${_FS}.vhd
+AZURE_IMG_${_FS}=	${.OBJDIR}/azure.${_FS}.vhdf
 AZURE_TARGET_${_FS}=	${OSRELEASE}-${_FS}${SNAPSHOT_DATE}
 
 azure-do-upload-${_FS}: