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

(cherry picked from commit 4194bbb34c60a5bf0fd0ba28f7bfa20b7842ba3f)