release: Report disk image filename

For someone new to the release bits it's not always clear what files are
being created. Report the disk image name explicitly.

Reviewed by:	gjb
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D39953

release/tools/vmimage.subr

@@ -318,6 +318,8 @@ vm_create_disk() {
 		${ROOTFSPART} \
 		-o ${VMIMAGE}
 
+	echo "Disk image ${VMIMAGE} created."
+
 	if [ ${ESP} = "yes" ]; then
 		rm ${espfilename}
 	fi